Time Machine backups to external drive keep failing

Again, the issue appears that TM is finding some files on your Mac as corrupted or missing, since the last successful backup. One of the first thing TM does is create a local snapshot of your Mac on your Mac's local drive. It looks like in this case, it couldn't do that.

What may be a better path to take is to actually verify your Mac's internal drive for any errors. You won't be able to do so when logged in. Instead you will need to reboot your Mac into Recovery Mode, and then, run the Disk Utility's First Aid option to check your Mac's internal drive's integrity.

Ref:

• Use macOS Recovery on a Mac with Apple silicon - Apple Support
• How to check and repair storage in Disk Utility - The Eclectic Light Company

If any of this seems "over your head," I would suggest that you contact Apple Support directly for further assistance with this issue. To do so, simply select the "Support" link on the lower left-hand side of this forum's web page.

It looks like a single file is either missing or corrupted that is preventing Time Machine (TM) from successfully backing up your Mac. That long filename provides you with a clue to where this file is located. That is, you would find it at: /System/Library/AssetsV2. Most likely it relates to an iPhone backup to your Mac.

With that said, you can try to just delete the specific file/folder first. If unable to, then the following would be the steps to remove the inProgress file and check the entire WD drive for any corruption before trying to start another manual TM backup.

STEP 1: Eject the WD drive

1. Eject the WD properly by right-clicking on the drive, and then, select Eject "Disk."
2. Then, unplug it, wait 10–15 seconds, and then, plug it back in. Let it mount fully, but don’t start a manual backup yet.
3. Optionally, you can remove this drive from TM.

STEP 2: Delete the inProgress file

The next step is to delete the stuck 'inProgress' file using the Terminal app. If you are uncomfortable using commands in the Terminal, you can stop here, and go on to STEP 3. However, if you are willing to give it a go, these are the steps to do so:

1. Run the Terminal app. It is located in the /Applications/Utilities folder on your Mac.
2. A Terminal window should appear with a command prompt. At that prompt, enter the following command, but DO NOT press the return key yet: sudo rm -rf
3. Drag the .inProgress file from the external drive into the Terminal window (this fills in the full path).
4. Now press Return/Enter.
5. Enter your Mac’s admin password when prompted — you won’t see anything as you type — then hit Return again. This should force-delete the file that Finder couldn't deal with. If this fails again, there may be underlying filesystem damage we’ll address next.

STEP 3: Run First Aid on the External Drive

1. We want to check to see if your WD drive's integrity. Start by running the Disk Utility app. It is located in the /Applications/Utilities folder.
2. Select your WD Passport from the sidebar.
3. Click First Aid and run it.
4. Let it run to completion. If errors are found and repaired, that might explain the stalling.

STEP 4: Reconnect to Time Machine

1. Go to System Settings > General > Time Machine.
2. Re-add the WD Passport if you removed it earlier.
3. Start a manual backup and watch for any error messages.

Although we could widen the exclusion list, this will eventually make TM backups somewhat useless.

The other option would be to delete all current TM local snapshots. These are created and maintained by TM during the backup process. It could be possible one or more of them is corrupted and causing this issue.

These are located on your Mac's internal drive. Again, they are maintained by TM and are periodically purged. You can see a list of them by using the Disk Utility. This is also where you can delete them.

Ref: View APFS snapshots in Disk Utility on Mac - Apple Support

Removing the inProgress file is not critical at this stage so we can ignore it for now. This leaves us with the files on your Mac that TM is refusing to back up.

What you're seeing is Time Machine choking on a system asset, specifically a file under /System/Library/AssetsV2, which is managed by macOS and often related to things like phone number lookups or other background iOS/macOS features. These types of files are not critical for backup, nor are they user data—but TM is still attempting to back them up, and for some reason (often permissions-related or due to file system protections), it's failing.

Since the error message is related to a macOS system file, let's make life a little easier by just excluding it from TM backups altogether.

To do so:

1. Go to System Settings > General > Time Machine.
2. Click Options.
3. Under “Exclude these items from backups,” click the + and navigate to: /System/Library/AssetsV2/com_apple_MobileAsset_phoneNumberResolver/
4. Add it, click Done, and try your backup again.

Remove and Re-add Drive in Time Machine

Go to System Settings > Time Machine.

Remove the WD Passport as a backup disk.

Reboot your iMac.

Reconnect the drive and re-add it in Time Machine preferences.

Whether or not, your Mac still has a Fusion drive, it doesn't affect TM from making snapshots. Interesting that yours doesn't show any. This would be typical if TM is not enabled.

Well, since there aren't any does rule out that snapshots would be an issue.

Sorry, this leaves me with nothing else to offer, other than completely erasing your Mac's internal drive, and the, re-installing a fresh copy of macOS. You also have the option to contact Apple Support directly BEFORE attempting to do so, especially if you have never done this yourself.