Time Machine error message and unable to backup to Ext HDD

[AMSOS]

I have never had a problem with Time Machine backups. But, now I am seeing this error message

"Time machine couldn't complete the backup. The backup disk couldn't be found..."

There is a red exclamation mark with the name of the ext HDD in Time Machine.

Now, the ext loads properly and I am able to access folders and files just fine. My other backup drive works as always.

I noticed last night that the automatically backup option had been turned off--likely by me--a month or so ago. Today, after restarting the computer and leaving it plugged in for some time, I noticed it had backed up properly and the error message had gone.

It all seems fine now, but is it? This is important data and I am a little worried. Can I carry out some diagnostic tests? Anything else?

Thanks!

 

[Brian33]

In my opinion, a problem that is described by “The backup disk couldn't be found..." probably would not have caused any problems with data already backed up, and I think the recently completed backup is probably fine. However, I do use both TM and Carbon Copy Cloner just in case TM messes up...

There is a way to “ Verify backups” in TM. What this does is to compare a newly-computed checksum of the file in the backup set to the value that was computed during its original back up. This will tell you if the file on the back up disk has changed or been corrupted since the time it was copied. Note that this process does not compare a file on the back up disk to the “original” file that may (or may not) exist on your boot disk!

This process may take a long time. I think you start the back up verification by holding down the alt key while clicking on the time machine icon in the menu bar and selecting it from the menu.