This isn’t a question of a previously working backup failing - the problem is that when I try to select the new drive, Time Machine instead selects the old drive unless I power it down, because it can’t tell the difference between the two drives.
Old backup was to drive named “Storage.”
I wish to change the backup to drive named “Backup.”
I go into Time Machine preferences. I select “TimeMachine” on “Backup-backup”.
I log in when prompted; the server name on the prompt is “MyBookLive-tm”.
The login happens successfully, but Time Machine connects to the old drive - it reports 750 gigs used (the new drive is empty) and 1tb total storage (the new drive is 2tb).
If I turn off the old drive (“Storage”), then Time Machine connects to the new drive as expected. But I need it to work with both drives turned on.
I assume what’s going on here is that the Time Machine share is being provided by something like the old UNIX netatalk software, and that the configuration file that provides the server name (“MyBookLive-tm”) is identical on both drives. If I can change that name, I’m about 99% sure the problem will go away.