Time Machine stopped working with WD My Cloud NAS drives "could not be accessed (error 16)"

Please start from here:

Backup disks you can use with Time Machine - Apple Support

then goto the option that you have chosen for your own on-premise site, to get more info if you've missed earlier.

DO NOT change account or password in NAS,etc drives.

while NAS drives are ON,

turn off all mac computers except one,

turn-off File-Sharing in that specific one mac computer,

& shutdown your that specific mac computer,

restart only one, that specific mac computer back ON,

enable file-sharing in that specific mac computer,

see if it can access NAS drive as Time-machine or not.

if it can its solved, you can repeat the process in other computers.

if above does not work ...

Then just change the access password (inside NAS,etc storage solution).

Back up your Mac with Time Machine - Apple Support

then re-connecting / re-accessing the NAS, etc storage drives back into macOS Time Machine settings

... should solve it.

Eli Sherer wrote:

we even had a power loss that assured everything was restarted a few times

Any kind of server or backup device should be on an uninterruptible power supply. An unexpected power loss can easily corrupt a hard drive. Network Time Machine volumes add another couple of levels of complexity, making them even more vulnerable.

Just delete the existing backup data and rebuild the drive.

It doesn't really matter how it happened. It still sounds like the volumes are corrupt. These things are sparse bundles and the Mac really isn't great about not corrupting network files to begin with. Delete the files and rebuild the backup.

Eli Sherer wrote:

<sigh> Not sure really how to do that... I'm guessing through Terminal?

Somewhere on the NAS. You may be able to access it through Terminal, via ssh. You might be able to access it by manually mounting the Time Machine share on the NAS. I'm not sure about that though. The system may detect that it is a Time Machine volume and prevent you from damaging it, which you actually do need to do in this case.

The issue here is that I have other files on the drives in a regular "public" partition. So reformatting is not an option.

I'm not sure how your NAS in configured, so I can't give specifics. Generally speaking, it is a very bad idea to ever store non-Time Machine files on a Time Machine volume for just this reason. Pretty much any problem in Time Machine can be fixed in seconds, and then a couple of hours to rebuild. But when you store other files there, now you have to find a place for those files. And if you were re-using Time Machine to avoid using another drive, you also have to get another drive to move those files to.

In your specific case, if you are accessing that Time Machine volume completely over the network, as I think you are, then you can avoid what I described above. You may be able to go into the directory where those Time Machine volumes are kept and delete just the spare bundle, leaving other files intact.

Not to mention that one laptop DOES have a valid sparsebundle that's still working. It's only the other two machines that cannot backup there...

Then don't delete the spare bundle file that is working. Just delete the two that are corrupt. Hopefully you have named them something recognizable and unique. If not, maybe you can distinguish them from the date. Look inside the sparse bundle and look for the most recent date. That should be the date of the last successful backup, or perhaps just the last backup attempt.

In the future, if you are networking these machines, make sure they have unique and recognizable names. You can set this in the System Preferences > Sharing. If you are using Ventura, <sigh> there is a new location for this in System Settings > General > About > Name. The Sharing location still exists, but it's buggy due to the new one that is added. Use the new one. Otherwise, they can get out of sync, and nobody knows what will happen then.