Not able to add a backup disk to time machine. Ventura.

pblumen wrote:

I have used this same drive successfully in the past with an old MBP. Maybe I should store the old back up someplace, reformat this drive and start over?

I think that is causing your problem, the old format of the drive and the fact that it had other files (or backups) on it -- erase the drive (at the highest device level), reformat it as APFS, then it will be available for Time Machine. If there is something on the drive you want to preserve, copy it to a different drive first, before erasing it.

In order for Time Machine to accept the drive it MUST be formatted using APFS, not HFS+. With Ventura a Time Machine volume can only be used for Backup and nothing else. It becomes a read-only volume and you can't store anything else on it.

The format on the drive has no bearing on this. Time machine will format it however it needs. If it is encrypted, then you’re going to need to decrypt it and mount it. It will not see an encrypted drive that is not mounted.

it will completely erase the drive.

What is the type, make and model of the drive?

If not an SSD is it a self powered drive?

How is it connected to the MBP?

Is it connected directly to the MBP or thru a dock?

If a dock is it a self powered dock?

pblumen wrote:

Suddenly not able to add a backup disk for use with Time Machine.

ref:

Back up your Mac Back up your Mac with Time Machine - Apple Support

What happens when you try? How are you trying to add the drive?

That more often than no jump starts Time Machine into working again. Be aware that the first backup can take a looooon time as it's copying every file instead of those changed and new files.