Time Machine failing to complete backup - Failed to unmount local snapshot?

I have an external USB drive to which I have been doing my Time Machine Backups for about 5 years now. Recently, it started refusing to complete the backup.

When I connect the drive and choose to create a backup, it either:

1. Stay in "Preparing backup" state for a long time and then goes back to idle (does not start the backup at all)
2. Starts the backup, after a couple of GBs it goes back to idle (as if nothing happened)

I tried scanning the disk with

fsck

, which reported everything being fine. I have also tried multiple other solutions, icluding removing the

.inProgress

file and rebooting the mac (this one looked promising, started preparing the backup and ran for about 5/30 GB, only to stop "back to idle" then).

I do see my previous backups listed on the disk with Time Machine, just creating the new ones seems to be a problem. It should not be disk space, as there is 250GB available (and the entire Mac's disk is 64 gigs).

After the backup fails, I see in its log:

Copying items from "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: 031D3803-B01C-3414-9912-9C615E0AE313 eventDBUUID: C58D9E07-C0C6-44F6-B27F-522C5C3895A7) 26 c :ERROR:-128, copy /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/namehere/2017-11 -05-133152/Macintosh HD to /Volumes/Time Machine/Backups.backupdb/namehere/2017-11-05-133153.inProgress/FB9A54C7-8DC5-40 1C-ABE0-7131A79EF624

Finished copying items for "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: 031D3803-B01C-3414-9912-9C615E0AE313 eventDBUUID: C58D9E07-C0C6-44F6-B27F-522C5C38 95A7) Time elapsed: 22 minutes, 58.000 seconds Copied 180065 items (5.69 GB) Copy stage canceled Backup canceled. Total time elapsed: 44 minutes, 26.000 seconds

After the first retry, the log looked similar (this time copied about 3 GB). Finally, trying to run the backup once again, resulted in the following log:

Running preflight for "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: 031D3803-B01C-3414-9912-9C615E0AE313 eventDBUUID: C58D9E07-C0C6-44F6-B27F-522C5C3895A7) Scanning nodes needing deep traversal Deep event scan at path: /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/namehere/2017-11 -05-191243/Macintosh HD reason:must scan subdirs|new event db| Calculating size of changes Backup canceled during preflight. Should copy 0 items (0 KB) representing 0 blocks of size 4096. 49066159 blocks available. Preflight complete for "Macintosh HD" (device: /dev/disk1s1 mount: '/' fsUUID: 031D3803-B01C-3414-9912-9C615E0AE313 eventDBUUID: C58D9E07-C0C6-44F6-B27F-522C5C3895A7) Time elapsed: 2 minutes, 28.000 seconds Backup canceled. Total time elapsed: 2 minutes, 31.000 seconds

In Console logs, I see:

17:01:34.124199 +0100 powerd Process backupd.624 TurnedOff BackgroundTask "Time Machine backup" age:00:16:03 id:47244673428 [System: PrevIdle DeclUser kDisp] 17:01:34.126417 +0100 powerd Process backupd.624 TurnedOff PreventUserIdleSystemSleep "Time Machine backup" age:00:16:03 id:4295000469 [System: PrevIdle DeclUser kDisp]

20:55:21.690979 +0100 backupd Claim 954EE5FE-C4AA-481E-8DE2-3B7462EEFDF5 was revoked

20:55:25.099189 +0100 backupd Failed to unmount disk mounted at '/Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/namehere/2017-1 1-13-202006/Macintosh HD', error: { Action = Unmount; Target = "file:///Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/namehere /2017-11-13-202006/Macintosh%20HD"; }

20:55:25.101240 +0100 backupd Failed to unmount snapshot: com.apple.TimeMachine.2017-11-13-202006 source: Macintosh HD

20:55:25.376956 +0100 powerd Process backupd.257 TurnedOff BackgroundTask "Time Machine backup" age:00:35:41 id:47244673059 [System: PrevIdle DeclUser kDisp] 20:55:25.378105 +0100 powerd Process backupd.257 TurnedOff PreventUserIdleSystemSleep "Time Machine backup" age:00:35:41 id:4295000100 [System: PrevIdle DeclUser kDisp]

20:55:42.788218 +0100 backupd Received claim 70B146B7-2002-4C35-B235-F23A6F07EAD7 20:55:42.789623 +0100 backupd Claim 70B146B7-2002-4C35-B235-F23A6F07EAD7 granted in server 20:55:42.789715 +0100 backupd Claim 70B146B7-2002-4C35-B235-F23A6F07EAD7 invoked in server

It seems that Time Machine fails to unmount a local snapshot?

I have tried running backup in safe mode, but with no results either. What could I do to complete a backup again?