How to choose a Time Machine backup disk?

Question

I am using macOS 12.6.2 (Monterey) and I have two backup disks configured in Time Machine. One is an old Airport Time Capsule that's on the local network, so it gets automatic backups every hour if my laptop is plugged in. Those are slow, and I'm worried that the drive is no longer very reliable, because the "overheating" light has been turned on for 2-3 years.

The second one is an external drive that I plug in using USB about once a week. It is quite a bit faster, but inconvenient since I normally use my laptop on my lap, and I don't want to carry around the external drive.

The Time Capsule is listed first in the Time Machine preferences page.

The problem is that if I plug in the external drive and say "Back up now", it almost always chooses to back up to the Time Capsule instead of the external drive, even if the most recent backup is much older on the external drive. I have to wait through the Time Capsule back up before saying "Back up now" again and getting my external backup.

Is there a way to change this? I'd like to be able to choose which drive to use if both are available, or always default to the external drive if it is plugged in.

Answer 1

I have a partial answer to my question using the tmutil utility in the terminal.

If I run

tmutil destinationinfo

I see results like this:

> ==================================================
Name          : Data
Kind          : Network
URL           : afp://...
Mount Point   : /Volumes/Time Machine Backups
ID            : 612...
====================================================
Name          : Time Machine 2
Kind          : Local
Mount Point   : /Volumes/Time Machine 2
ID            : A50...

The external drive is the second one, so to trigger an immediate backup to that drive I can type

tmutil startbackup --destination A50...

and this seems to work. The ID values are long hex strings (about 30 hex digits), so I'll need to create an alias or script file to run this. If anyone can suggest something that works using the regular menu interface, I'd prefer that.