diff --git a/CHANGELOG.md b/CHANGELOG.md
index b93f283..cbbfe87 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -48,6 +48,10 @@ instead carried by the app description, the git tag, the release, and this file.
   overwriting the result. The lease now lasts at least 15 minutes, a step writes the run only if
   no other process wrote it since the step loaded it (in one database statement), and recording
   a snapshot that is already recorded is no longer an error.
+- `occ backup:verify` on a large repository failed with "SFTP connection failed while opening
+  snapshots/…": after downloading the index it refreshed the local index cache with a pass over
+  all blobs for every pack, which took 29 minutes for 1 TB (27,547 packs, 400,000 blobs). The
+  location dropped the idle connection in the meantime. It is one pass now (under a second).
 
 ## [0.9.4-beta.5] - 2026-10-09
 
diff --git a/lib/Backend/SftpReadStream.php b/lib/Backend/SftpReadStream.php
index 1c9ff13..751b900 100644
--- a/lib/Backend/SftpReadStream.php
+++ b/lib/Backend/SftpReadStream.php
@@ -67,6 +67,8 @@ final class SftpReadStream {
 	public const MAX_FAILURES = 3;
 	/** Set by stream_open() when the connection failed rather than the file. */
 	private static bool $openBroke = false;
+	/** Why the last stream_open() failed on the connection. */
+	private static string $openCause = '';
 
 	/** @var resource|null set by PHP for stream wrappers */
 	public $context;
@@ -135,7 +137,7 @@ final class SftpReadStream {
 			return null;
 		}
 		if ($fh === false && self::openBroke()) {
-			throw new BackendException('SFTP connection failed while opening ' . $path);
+			throw new BackendException('SFTP connection failed while opening ' . $path . ': ' . self::$openCause);
 		}
 		return $fh;
 	}
@@ -339,14 +341,14 @@ final class SftpReadStream {
 			}
 			$id = self::nextId();
 			if (!$this->sftp->_send_sftp_packet(self::FXP_OPEN, pack('Na*N2', strlen($remote), $remote, self::FXF_READ, 0), $id)) {
-				return $this->connectionFailed();
+				return $this->connectionFailed('the request could not be sent');
 			}
 			$response = $this->sftp->_get_sftp_packet($id);
-		} catch (\Throwable) {
-			return $this->connectionFailed();
+		} catch (\Throwable $e) {
+			return $this->connectionFailed(get_class($e) . ': ' . $e->getMessage());
 		}
 		if (!is_string($response)) {
-			return $this->connectionFailed();
+			return $this->connectionFailed('no answer');
 		}
 		if ($this->sftp->packet_type !== self::FXP_HANDLE) {
 			return false; // an answer, but no handle: the file cannot be opened
@@ -356,12 +358,37 @@ final class SftpReadStream {
 	}
 
 	/** Not a missing file but a connection that does not behave as expected. */
-	private function connectionFailed(): bool {
+	private function connectionFailed(string $cause): bool {
 		self::$failures++;
 		self::$openBroke = true;
+		self::$openCause = $cause . self::connectionState($this->sftp);
 		return false;
 	}
 
+	/** What phpseclib knows about the connection, for an error message. */
+	private static function connectionState(object $sftp): string {
+		$state = [];
+		try {
+			if (is_callable([$sftp, 'isConnected'])) {
+				$state[] = 'connected: ' . ($sftp->isConnected() ? 'yes' : 'no');
+			}
+			if (is_callable([$sftp, 'getLastError'])) {
+				$last = (string)$sftp->getLastError();
+				if ($last !== '') {
+					$state[] = 'ssh: ' . $last;
+				}
+			}
+			if (is_callable([$sftp, 'getLastSFTPError'])) {
+				$last = (string)$sftp->getLastSFTPError();
+				if ($last !== '') {
+					$state[] = 'sftp: ' . $last;
+				}
+			}
+		} catch (\Throwable) {
+		}
+		return $state === [] ? '' : ' (' . implode('; ', $state) . ')';
+	}
+
 	/** Whether the last stream_open() failed on the connection; a call, as fopen() sets it. */
 	private static function openBroke(): bool {
 		return self::$openBroke;
diff --git a/lib/Repository/BlobIndex.php b/lib/Repository/BlobIndex.php
index a71faa6..ec05c55 100644
--- a/lib/Repository/BlobIndex.php
+++ b/lib/Repository/BlobIndex.php
@@ -126,14 +126,15 @@ final class BlobIndex {
 		}
 		if ($verify && $this->cache !== null) {
 			// Refresh the cache from what was just verified.
+			// One pass over the blobs: a pass per pack took half an hour on a 1 TB repository
+			// (27,547 packs x 400,000 blobs), long enough for the location to drop the idle
+			// connection before the check went on.
+			$byPack = [];
+			foreach ($this->blobs as $id => [$pack, $offset, $length, $raw, $flags]) {
+				$byPack[$pack][] = ['id' => $id, 'offset' => $offset, 'length' => $length, 'raw' => $raw, 'flags' => $flags];
+			}
 			$this->cache->remove($this->repositoryId, array_keys($this->cache->all($this->repositoryId)));
-			foreach ($this->packs() as $packId => $_) {
-				$entries = [];
-				foreach ($this->blobs as $id => [$pack, $offset, $length, $raw, $flags]) {
-					if ($pack === $packId) {
-						$entries[] = ['id' => $id, 'offset' => $offset, 'length' => $length, 'raw' => $raw, 'flags' => $flags];
-					}
-				}
+			foreach ($byPack as $packId => $entries) {
 				$this->cache->put($this->repositoryId, $packId, $entries);
 			}
 			return;