Time Machine can't back up to Airport Time Capsule

If you review my comments, I did not say that you needed to replace the Time Capsule, I said that it would be a good idea to also be backing up to another drive that is connected directly to your Mac.

It would be a very simple matter to connect a USB drive to your Mac and back up that way in addition to backing up to the Time Capsule. If you do this, Time Machine will alternate backing up to the Time Capsule and the USB drive.

Then, you will have backups on two different drives. If the Time Capsule fails, you at least have a backup history on the USB drive.

I'm using an iMac with MacOS Catalina (10.15.7).

This day was inevitable. Since Catalina made a bad problem worse.. you have escaped until now the day of reckoning.. it has finally arrived.

The backup disk is not in Mac OS Extended (Journaled) format, which is required.

The error message is, of course, utter nonsense.

Have you done the usual..

1 reboot the TC (Time Capsule)?

If that doesn't help

2 reboot the entire network including the computer and TC?

If that doesn't help

3 reset the TC to factory. No files are lost. Reconfigure it from airport utility on the errant Mac.

Use short name, no spaces and pure alphanumerics. Passwords likewise but 8-20 or more characters.

You will need to reselect the TC as the Time Machine (TM) target.

TM will run a verify after you setup again of the existing backup.. if it fails or is corrupted it will give you a message .. otherwise it will run the next incremental .. this can take a long time.

If that doesn't help

4 You will need to make a big decision. A 2014 TC is now 6 years old and well and truly past its use by date. It is time to reconsider backup.

For an iMac I would simply plug a USB drive into port on the back and use that as your Time Machine target. Local backup is faster, more reliable and much cheaper than any network alternative.

If that doesn't help

5 You can erase the TC and start over. If you wish to preserve the backup you will need a USB drive of size equal to the TC Mac HFS+ formatted and plugged into the TC.. select archive in the airport utility disk tab.

Once completed, select erase.

A quick erase will merely delete the indexes.. and takes a min or two.

You can also choose a full erase. Zero out data. This will take several hours and will do low level format of the drive.. testing all the tracks.. well worthwhile after 6 years on the job.

Since TM on Catalina is known to be problematic with the TC I would strongly recommend using alternative like Carbon Copy Cloner.

Hopefully, the restart will get things working again.

But, even if they do, you would not want to keep data on the Time Capsule that you cannot afford to lose......unless.....you are also backing up to another hard drive.

At 6 years, the hard drive inside the Time Capsule is really suspect. At 7 years + it is overdue to fail.

2 empty Thunderbolt ports that I could put to good use

Thunderbolt external drives are expensive.

Any Mac since 2013 should have USB3 ports .. USB is much cheaper and much faster than standard mechanical drives so more than adequate for backups. For most people even USB2 on a computer works fine for backup.

Thunderbolt are suitable for SSD external used for super fast storage or even alternative boot drive.

You can certainly get thunderbolt drive enclosures but expect to pay loads for them.