diff --git a/apps/files_external/css/settings.css b/apps/files_external/css/settings.css
index 6dfb012b15eb..cc924db27987 100644
--- a/apps/files_external/css/settings.css
+++ b/apps/files_external/css/settings.css
@@ -102,3 +102,13 @@ td.mountPoint, td.backend { width:160px; }
 #externalStorage .mountOptionsDropdown {
 	margin-right: 40px;
 }
+
+#externalStorage td.configuration .error-invalid,
+#externalStorage td.authentication .error-invalid {
+	white-space: normal;
+
+}
+
+#externalStorage tr.invalid {
+	background-color: #F2DEDE;
+}
diff --git a/apps/files_external/js/settings.js b/apps/files_external/js/settings.js
index 933e1ec01d70..0794a77e2bf9 100644
--- a/apps/files_external/js/settings.js
+++ b/apps/files_external/js/settings.js
@@ -809,6 +809,19 @@ MountConfigListView.prototype = _.extend({
 	newStorage: function(storageConfig, onCompletion) {
 		var mountPoint = storageConfig.mountPoint;
 		var backend = this._allBackends[storageConfig.backend];
+		var isInvalidAuth = false;
+
+		if (!backend) {
+			backend = {
+				name: 'Unknown backend: ' + storageConfig.backend,
+				invalid: true
+			};
+		}
+		if (backend && storageConfig.authMechanism && !this._allAuthMechanisms[storageConfig.authMechanism]) {
+			// mark invalid to prevent editing
+			backend.invalid = true;
+			isInvalidAuth = true;
+		}
 
 		// FIXME: Replace with a proper Handlebar template
 		var $tr = this.$el.find('tr#addMountPoint');
@@ -836,6 +849,26 @@ MountConfigListView.prototype = _.extend({
 		$tr.addClass(backend.identifier);
 		$tr.find('.backend').data('identifier', backend.identifier);
 
+		if (backend.invalid || isInvalidAuth) {
+			$tr.addClass('invalid');
+			$tr.find('[name=mountPoint]').prop('disabled', true);
+			$tr.find('.applicable,.mountOptionsToggle').empty();
+			this.updateStatus($tr, false, 'Unknown backend: ' + backend.name);
+			if (isInvalidAuth) {
+				$tr.find('td.authentication').append('<span class="error-invalid">' +
+					t('files_external', 'Unknown auth backend "{b}"', {b: storageConfig.authMechanism}) +
+					'</span>'
+				);
+			}
+
+			$tr.find('td.configuration').append('<span class="error-invalid">' +
+				t('files_external', 'Please make sure that the app that provides this backend is installed and enabled') +
+				'</span>'
+			);
+
+			return $tr;
+		}
+
 		var selectAuthMechanism = $('<select class="selectAuthMechanism"></select>');
 		var neededVisibility = (this._isPersonal) ? StorageConfig.Visibility.PERSONAL : StorageConfig.Visibility.ADMIN;
 		$.each(this._allAuthMechanisms, function(authIdentifier, authMechanism) {
diff --git a/apps/files_external/lib/Command/ListCommand.php b/apps/files_external/lib/Command/ListCommand.php
index 503c62af1368..15642ff789bd 100644
--- a/apps/files_external/lib/Command/ListCommand.php
+++ b/apps/files_external/lib/Command/ListCommand.php
@@ -34,6 +34,8 @@
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Input\InputOption;
 use Symfony\Component\Console\Output\OutputInterface;
+use OCP\Files\External\Backend\InvalidBackend;
+use OCP\Files\External\Auth\InvalidAuth;
 
 class ListCommand extends Base {
 	/**
@@ -192,7 +194,11 @@ public function listMounts($userId, array $mounts, InputInterface $input, Output
 				'enable_sharing' => false,
 				'encoding_compatibility' => false
 			];
-			$rows = array_map(function (IStorageConfig $config) use ($userId, $defaultMountOptions, $full) {
+			$countInvalid = 0;
+			$rows = array_map(function (IStorageConfig $config) use ($userId, $defaultMountOptions, $full, &$countInvalid) {
+				if ($config->getBackend() instanceof InvalidBackend || $config->getAuthMechanism() instanceof InvalidAuth) {
+					$countInvalid++;
+				}
 				$storageConfig = $config->getBackendOptions();
 				$keys = array_keys($storageConfig);
 				$values = array_values($storageConfig);
@@ -256,6 +262,14 @@ public function listMounts($userId, array $mounts, InputInterface $input, Output
 			$table->setHeaders($headers);
 			$table->setRows($rows);
 			$table->render();
+
+			if ($countInvalid > 0) {
+				$output->writeln(
+					"<error>Number of invalid storages found: $countInvalid.\n" .
+					"The listed configuration details are likely incomplete.\n" .
+					"Please make sure that all related apps that provide these storages are enabled or delete these.</error>"
+				);
+			}
 		}
 	}
 
diff --git a/apps/files_external/tests/Command/ListCommandTest.php b/apps/files_external/tests/Command/ListCommandTest.php
index e1a279fdd841..97d89481a0e5 100644
--- a/apps/files_external/tests/Command/ListCommandTest.php
+++ b/apps/files_external/tests/Command/ListCommandTest.php
@@ -31,6 +31,7 @@
 use OC\Files\External\StorageConfig;
 use OCP\Files\External\IStorageConfig;
 use Symfony\Component\Console\Output\BufferedOutput;
+use OCP\Files\External\Backend\InvalidBackend;
 
 class ListCommandTest extends CommandTest {
 	/**
@@ -71,4 +72,26 @@ public function testListAuthIdentifier() {
 
 		$this->assertNotEquals($output[0]['authentication_type'], $output[1]['authentication_type']);
 	}
+
+	public function testDisplayWarningForIncomplete() {
+		$l10n = $this->createMock('\OCP\IL10N', null, [], '', false);
+		$session = $this->createMock('\OCP\ISession');
+		$crypto = $this->createMock('\OCP\Security\ICrypto');
+		$instance = $this->getInstance();
+		// FIXME: use mock of IStorageConfig
+		$mount1 = new StorageConfig();
+		$mount1->setAuthMechanism(new Password());
+		$mount1->setBackend(new InvalidBackend('InvalidId'));
+		$mount2 = new StorageConfig();
+		$mount2->setAuthMechanism(new SessionCredentials($session, $crypto));
+		$mount2->setBackend(new Local($l10n, new NullMechanism()));
+		$input = $this->getInput($instance);
+		$output = new BufferedOutput();
+
+		$instance->listMounts('', [$mount1, $mount2], $input, $output);
+		$output = $output->fetch();
+
+		$lines = explode($output, "\n");
+		$this->assertRegexp('/Number of invalid storages found/', $output);
+	}
 }
diff --git a/lib/private/Files/External/InvalidStorage.php b/lib/private/Files/External/InvalidStorage.php
new file mode 100644
index 000000000000..f397fc1fbcb4
--- /dev/null
+++ b/lib/private/Files/External/InvalidStorage.php
@@ -0,0 +1,31 @@
+<?php
+/**
+ * @author Vincent Petry <pvince81@owncloud.com>
+ *
+ * @copyright Copyright (c) 2017, ownCloud GmbH.
+ * @license AGPL-3.0
+ *
+ * This code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License, version 3,
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ *
+ */
+
+namespace OC\Files\External;
+
+use OCP\Files\StorageNotAvailableException;
+use OC\Files\Storage\FailedStorage;
+
+class InvalidStorage extends FailedStorage {
+	public function __construct($params) {
+		throw new StorageNotAvailableException();
+	}
+}
diff --git a/lib/private/Files/External/Service/StoragesService.php b/lib/private/Files/External/Service/StoragesService.php
index 88859e3a40d2..9485fbf8c799 100644
--- a/lib/private/Files/External/Service/StoragesService.php
+++ b/lib/private/Files/External/Service/StoragesService.php
@@ -37,6 +37,8 @@
 use OCP\Files\External\IStoragesBackendService;
 use OCP\Files\External\NotFoundException;
 use OCP\Files\External\Service\IStoragesService;
+use OCP\Files\External\Backend\InvalidBackend;
+use OCP\Files\External\Auth\InvalidAuth;
 
 /**
  * Service class to manage external storages
@@ -307,11 +309,11 @@ public function createStorage(
 	) {
 		$backend = $this->backendService->getBackend($backendIdentifier);
 		if (!$backend) {
-			throw new \InvalidArgumentException('Unable to get backend for ' . $backendIdentifier);
+			$backend = new InvalidBackend($backendIdentifier);
 		}
 		$authMechanism = $this->backendService->getAuthMechanism($authMechanismIdentifier);
 		if (!$authMechanism) {
-			throw new \InvalidArgumentException('Unable to get authentication mechanism for ' . $authMechanismIdentifier);
+			$authMechanism = new InvalidAuth($authMechanismIdentifier);
 		}
 		$newStorage = $this->createConfig();
 		$newStorage->setMountPoint($mountPoint);
@@ -389,11 +391,15 @@ public function updateStorage(IStorageConfig $updatedStorage) {
 		$existingMount = $this->dbConfig->getMountById($id);
 
 		if (!is_array($existingMount)) {
-			throw new NotFoundException('Storage with id "' . $id . '" not found while updating storage');
+			throw new NotFoundException('Storage config with id "' . $id . '" not found while updating storage');
 		}
 
 		$oldStorage = $this->getStorageConfigFromDBMount($existingMount);
 
+		if ($oldStorage->getBackend() instanceof InvalidBackend) {
+			throw new NotFoundException('Storage config with id "' . $id . '" and backend id "' . $oldStorage->getBackend()->getInvalidId() . '" cannot be edited due to missing backend');
+		}
+
 		$removedUsers = array_diff($oldStorage->getApplicableUsers(), $updatedStorage->getApplicableUsers());
 		$removedGroups = array_diff($oldStorage->getApplicableGroups(), $updatedStorage->getApplicableGroups());
 		$addedUsers = array_diff($updatedStorage->getApplicableUsers(), $oldStorage->getApplicableUsers());
diff --git a/lib/public/Files/External/Auth/InvalidAuth.php b/lib/public/Files/External/Auth/InvalidAuth.php
new file mode 100644
index 000000000000..52a3c71ddfc5
--- /dev/null
+++ b/lib/public/Files/External/Auth/InvalidAuth.php
@@ -0,0 +1,47 @@
+<?php
+/**
+ * @author Vincent Petry <pvince81@owncloud.com>
+ *
+ * @copyright Copyright (c) 2017, ownCloud GmbH.
+ * @license AGPL-3.0
+ *
+ * This code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License, version 3,
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ *
+ */
+
+namespace OCP\Files\External\Auth;
+
+/**
+ * Invalid authentication representing an auth mechanism
+ * that could not be resolved
+ *
+ * @since 10.0.5
+ */
+class InvalidAuth extends AuthMechanism {
+
+	/**
+	 * Constructs a new InvalidAuth with the id of the invalid auth
+	 * for display purposes
+	 *
+	 * @param string $invalidId invalid id
+	 *
+	 * @since 10.0.5
+	 */
+	public function __construct($invalidId) {
+		$this->setIdentifier($invalidId)
+			->setScheme(self::SCHEME_NULL)
+			->setText('Unknown auth mechanism backend ' . $invalidId)
+		;
+	}
+
+}
diff --git a/lib/public/Files/External/Backend/InvalidBackend.php b/lib/public/Files/External/Backend/InvalidBackend.php
new file mode 100644
index 000000000000..13eda1bb6dfd
--- /dev/null
+++ b/lib/public/Files/External/Backend/InvalidBackend.php
@@ -0,0 +1,64 @@
+<?php
+/**
+ * @author Vincent Petry <pvince81@owncloud.com>
+ *
+ * @copyright Copyright (c) 2017, ownCloud GmbH.
+ * @license AGPL-3.0
+ *
+ * This code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License, version 3,
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ *
+ */
+
+namespace OCP\Files\External\Backend;
+
+use \OCP\IL10N;
+
+/**
+ * Invalid storage backend representing a backend
+ * that could not be resolved
+ *
+ * @since 10.0.5
+ */
+class InvalidBackend extends Backend {
+
+	/** @var string Invalid backend id */
+	private $invalidId;
+
+	/**
+	 * Constructs a new InvalidBackend with the id of the invalid backend
+	 * for display purposes
+	 *
+	 * @param string $invalidId id of the backend that did not exist
+	 *
+	 * @since 10.0.5
+	 */
+	function __construct($invalidId) {
+		$this->invalidId = $invalidId;
+		$this->setIdentifier($invalidId)
+			->setStorageClass('\OC\Files\External\InvalidStorage')
+			->setText('Unknown storage backend ' . $invalidId)
+		;
+	}
+
+	/**
+	 * Returns the invalid backend id
+	 *
+	 * @return string invalid backend id
+	 *
+	 * @since 10.0.5
+	 */
+	public function getInvalidId() {
+		return $this->invalidId;
+	}
+}
+
diff --git a/tests/lib/Files/External/InvalidStorageTest.php b/tests/lib/Files/External/InvalidStorageTest.php
new file mode 100644
index 000000000000..19ee41ade997
--- /dev/null
+++ b/tests/lib/Files/External/InvalidStorageTest.php
@@ -0,0 +1,34 @@
+<?php
+/**
+ * @author Vincent Petry <pvince81@owncloud.com>
+ *
+ * @copyright Copyright (c) 2017, ownCloud GmbH.
+ * @license AGPL-3.0
+ *
+ * This code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License, version 3,
+ * as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License, version 3,
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>
+ *
+ */
+namespace Test\Files\External;
+
+use Test\TestCase;
+use OC\Files\External\InvalidStorage;
+
+class InvalidStorageTest extends TestCase {
+
+	/**
+	 * @expectedException OCP\Files\StorageNotAvailableException
+	 */
+	public function testCannotInstantiate() {
+		new InvalidStorage([]);
+	}
+}
diff --git a/tests/lib/Files/External/Service/StoragesServiceTest.php b/tests/lib/Files/External/Service/StoragesServiceTest.php
index 0164bfcae701..13be1a73a97f 100644
--- a/tests/lib/Files/External/Service/StoragesServiceTest.php
+++ b/tests/lib/Files/External/Service/StoragesServiceTest.php
@@ -31,6 +31,9 @@
 use OCP\Files\External\NotFoundException;
 use OCP\Files\External\IStoragesBackendService;
 use Test\TestCase;
+use OCP\Files\External\Backend\InvalidBackend;
+use OCP\Files\External\Auth\InvalidAuth;
+use OCP\Files\External\Backend\Backend;
 
 /**
  * @group DB
@@ -67,6 +70,11 @@ abstract class StoragesServiceTest extends TestCase {
 	 */
 	protected $mountCache;
 
+	/**
+	 * @var Backend[]
+	 */
+	protected $backends;
+
 	public function setUp() {
 		parent::setUp();
 		$this->dbConfig = new CleaningDBConfig(\OC::$server->getDatabaseConnection(), \OC::$server->getCrypto());
@@ -108,21 +116,24 @@ public function setUp() {
 
 		$sftpBackend = $this->getBackendMock('\OCA\Files_External\Lib\Backend\SFTP', '\OCA\Files_External\Lib\Storage\SFTP');
 		$dummyBackend = $this->getBackendMock('\Test\Files\External\Backend\DummyBackend', '\Test\Files\External\Backend\DummyStorage');
-		$backends = [
+		$this->backends = [
 			'identifier:\OCA\Files_External\Lib\Backend\SMB' => $this->getBackendMock('\OCA\Files_External\Lib\Backend\SMB', '\OCA\Files_External\Lib\Storage\SMB'),
 			'identifier:\OCA\Files_External\Lib\Backend\SFTP' => $sftpBackend,
 			'identifier:\Test\Files\External\Backend\DummyBackend' => $dummyBackend,
 			'identifier:sftp_alias' => $sftpBackend,
 		];
 		$this->backendService->method('getBackend')
-			->will($this->returnCallback(function ($backendClass) use ($backends) {
-				if (isset($backends[$backendClass])) {
-					return $backends[$backendClass];
+			->will($this->returnCallback(function ($backendClass) {
+				if (isset($this->backends[$backendClass])) {
+					return $this->backends[$backendClass];
 				}
 				return null;
 			}));
 		$this->backendService->method('getBackends')
-			->will($this->returnValue($backends));
+			->will($this->returnCallback(function () {
+				// in case they changed
+				return $this->backends;
+			}));
 
 		\OCP\Util::connectHook(
 			Filesystem::CLASSNAME,
@@ -349,27 +360,29 @@ public function testCreateStorage() {
 	}
 
 	/**
-	 * @expectedException \InvalidArgumentException
 	 */
 	public function testCreateStorageInvalidClass() {
-		$this->service->createStorage(
+		$storageConfig = $this->service->createStorage(
 			'mount',
 			'identifier:\OC\Not\A\Backend',
 			'identifier:\Auth\Mechanism',
 			[]
 		);
+
+		$this->assertInstanceOf(InvalidBackend::class, $storageConfig->getBackend());
+
 	}
 
 	/**
-	 * @expectedException \InvalidArgumentException
 	 */
 	public function testCreateStorageInvalidAuthMechanismClass() {
-		$this->service->createStorage(
+		$storageConfig = $this->service->createStorage(
 			'mount',
 			'identifier:\OCA\Files_External\Lib\Backend\SMB',
 			'identifier:\Not\An\Auth\Mechanism',
 			[]
 		);
+		$this->assertInstanceOf(InvalidAuth::class, $storageConfig->getAuthMechanism());
 	}
 
 	public function testGetStoragesBackendNotVisible() {
@@ -486,4 +499,28 @@ public function testUpdateStorageMountPoint() {
 		$this->assertEquals($newAuthMechanism, $savedStorage->getAuthMechanism());
 		$this->assertEquals('password2', $savedStorage->getBackendOption('password'));
 	}
+
+	/**
+	 * @expectedException OCP\Files\External\NotFoundException
+	 */
+	public function testCannotEditInvalidBackend() {
+		$backend = $this->backendService->getBackend('identifier:\Test\Files\External\Backend\DummyBackend');
+		$authMechanism = $this->backendService->getAuthMechanism('identifier:\Auth\Mechanism');
+
+		$storage = new StorageConfig();
+		$storage->setMountPoint('mountpoint');
+		$storage->setBackend($backend);
+		$storage->setAuthMechanism($authMechanism);
+		$storage->setBackendOptions(['password' => 'testPassword']);
+
+		$savedStorage = $this->service->addStorage($storage);
+
+		// make it invalid
+		$this->backends['identifier:\Test\Files\External\Backend\DummyBackend'] = new InvalidBackend('identifier:\Test\Files\External\Backend\DummyBackend');
+
+		$updatedStorage = new StorageConfig($savedStorage->getId());
+		$updatedStorage->setBackendOptions(['password' => 'password2']);
+
+		$this->service->updateStorage($updatedStorage);
+	}
 }
diff --git a/tests/lib/Files/External/Service/UserGlobalStoragesServiceTest.php b/tests/lib/Files/External/Service/UserGlobalStoragesServiceTest.php
index 85a8435e13d9..f048f3e12e51 100644
--- a/tests/lib/Files/External/Service/UserGlobalStoragesServiceTest.php
+++ b/tests/lib/Files/External/Service/UserGlobalStoragesServiceTest.php
@@ -366,4 +366,9 @@ public function testUpdateStorageMountPoint() {
 		// we don't test this here
 		$this->assertTrue(true);
 	}
+
+	public function testCannotEditInvalidBackend() {
+		// we don't test this here
+		$this->assertTrue(true);
+	}
 }