Time Machine setup issue after upgrading from MacBook Pro to Mac mini

If I interpret correctly, you decided to wipe your two old TM backups on a 1 TB and a 2 TB drive and start fresh, but the new backups are not working because they're only able to back up your external storage on the Mini.

I'm not sure how you went about that, but what you should have done (and if you didn't, what you should do) is use Disk Utility to erase and reformat the backup drives. Connect each drive, launch Disk Utility, select the drive then click Erase in the top tool bar. Set the partition scheme to GUID and the drive format to APFS. Do not use the encrypted format. If you want to encrypt the backups (which is a good idea), select that option in Time Machine.

Once you have the reformatted drives, connect one to your Mac, you may be prompted to use it as a TM backup but if not, then go to TM settings and click the (+) button to set up the drive.

FYI, the 2.5x internal storage is only a recommendation, TM will run the backup even if the destination drive is smaller, as long as there's space on the backup drive for all the data being backed up. I routinely back up several Macs to a pair of 4 TB SSDs where each points to a partition that is the same size as the internal storage, e.g. a 2 TB partition for the 2 TB MacBook Pro. The only real consequence of that is that the 'time' component will eventually be limited as TM starts to delete older backups to make room.

Amit Kher wrote:

the quirks of technology

yesterday it didn't work

today i tried again and it's working, only difference from earlier being Macintosh HD now appears as "Data"

That is where your data is stored. The volume named Macintosh HD is the OS, the "System" volume. The Data volume is separate from the System volume. You can no longer back up the System, only the data.

i tried adding another 2 tb disk, thinking i could use it for only Mac disk, but different settings for 2 Time Machines no longer work

i’m somewhat confused by this statement because I have no idea how you could set up Time Machine differently for two different backup discs. I don’t believe that would ever work now or in the past. You can set more than one backup drive, but the only thing it does is alternate between the drive. It makes the back up using all the same settings. Remove all drives from Time Machine and then delete the.com.apple.timemachine.plist from /Library/Preferences. Then, try to set it up again.

Deleting the preference file won’t hurt, it will start you from scratch and the file will just be recreated so I would go ahead and do that.