I have used Time Machine to backup my desktop. No problem. I can enter Time Machine and retrieve/restore any file. No problem.
What just happened today, however, is that that when I click on Time Machine TO DO a backup - manual backup, not automated - the backup disc cannot be found on the Time Capsule's drive. It IS there because I can retrieve files from it; but the drive is not found when I wish to do a backup. It also no longer appears in the Select Disc tab of the Time Machine menu.
I know the backup contents are still on the external drive, I just cannot back up to it.
RE: Have you tried clicking on Select Disk then clicking on the currently selected disk and then clicking on Remove Disk then selecting the disk again.
That is the problem. The iMAC backup disk does not show up as an option to SELECT DISK. The only computer which shows up there is the MBPro . . . but the MBPro has been completely erased and Mojave reinstalled. It is not going to be used anymore to backup to this Time Capsule.
All I am trying to do is find a way for the iMAC to recognize the backup it created on the Time Capsule using Time Machine. I have no problem locating the backups with Time Machine to RETRIEVE files back to the desktop computer, so the iMAC locates the backup for that process. However, Time Machine will not find the drive to DO a backup from the desktop computer.
I understand what the problem is, I am trying to put some boundaries on the solution space. Since you can recover files from the Time Capsule back to the iMac the iMac obviously knows where the backup data set resides and that should be reflected in System Preferences > Time Machine and assuming you have automatic backups enabled that should be happening once an hour and no reconnection should be necessary. Neither should a reconnection be required if you trigger a manual backup. Some of my questions were an attempt to determine IF the automatic backups are still occurring.
By the same token you should never have to locate or specify the backup data set file per. se. all you need to specify is the Time Capsule (or backup drive) and Time Machine is supposed to take it from there. I had not realized previously that you were backing up to a Time Capsule. In the case of a Time Capsule the backup data sets are computername.sparsebundle which is a variant of dmg and like a dmg can be "mounted" as if it were a physical device. To see the sparseimage files on your Time Capsule in Finder requires you to connect to the Time Capsule using the password for your Airport router. (In normal use Time Machine only mounts the sparseimage while it is being accessed.)
In any case the backup data set is just files and assuming your iMac is connected to the Time Capsule should be visible in Finder on either the iMac or MacBook Pro. Since the iMac backup is not appearing in Finder and the MBP backup is, I was wondering if somehow the iMac data set managed to get encapsulated under the MBP data set which some of my questions were intended to ferret out.