Mac – Time Machine asks for a backup disk by the wrong name

Question: Is the [AEBS + USB/AirDisk + Time Machine] combo finally, officially supported? On Mountain Lion or something? I couldn't find documentation along those lines but since we're talking about backups, I just want to make sure I'm not following an unsupported mechanism.

It was not supported as of Mountain Lion (see OS X Mountain Lion: Disks you can use with Time Machine) but according to the newest KB article (Use Time Machine to back up or restore your Mac) it is now officially supported:

Time Machine is the built-in backup feature of your Mac. To use it, you need an external storage solution, sold separately:

• External hard drive connected to a USB, FireWire, or Thunderbolt port on your Mac
• Time Capsule or macOS Server on your network
• External hard drive connected to the USB port of an AirPort Extreme base station on your network

I couldn't find any information about whether this functionality was added as an Airport Express firmware update or was introduced with a specific OS version (or both), and thus I can't provide a firmware/OS version support matrix, but from Backup disks you can use with Time Machine the minimum requirements seem to be:

• AirPort Extreme 802.11ac model
• Yosemite (OS X 10.10) (the earliest OS that references the article above, see OS X Yosemite: Back up your files with Time Machine)

Basically, I would recommend not trying to constrain time machine to only a weekly interval on the offsite drive.

That being said, you can have both drives work totally independently as that is how the software is engineered. If you really only want a weekly snapshot, only connect the weekly drive once a week and force a rotation of the destination:

 tmutil startbackup --rotation

Once you have a backup to the weekly drive, eject it and send it away. As long as all dives get reattached every 14 days or so, you won't get a warning dialog that any are out of date.

Again, you can certainly enforce a single weekly backup by the procedure you follow but two things make that practice less needed than you might expect:

• The backup intervals are very low space overhead compared to the size of cheap backup disks.

On my worst Mac, the system averages 5 MB of overhead for system log files so in absence of user files changing, you have 24 daily and 31 monthly snapshots you would avoid storing 275 MB (or 0.0003 of a one TB drive) if you were on a strict one per week schedule.

Since the cleanup of old backups is so fast and the storage impact so low, most clients with offsite rotation end up just swapping disks each week and let the software cull the extra snapshots per the normal schedule, leaving weekly snapshots more than a month old.