Back up Photos and iTunes libraries located on external disk

I would appreciate help in setting up a back system. I have a MacBook Air whose SSD is not large enough for photos and iTunes material. So, I have an external disk called Air Media for this material. I have a separate external disk for Time Machine. But I just noticed by default that the Time Machine settings default to excluding Air Media. If I remove that exclusion from the Time Machine settings would Air Media, which has the Photos Library and iTunes Library, be backed up by Time Machine such that I could use with another machine when needed. Air Media contains 700 GB of data, which appears about what I would expect. It seems like what I am proposing should work, but I cannot find confirmation when Internet searching.


Correct. External drives are assumed to be temporarily-attached and are excluded from TM backups by default. But you can remove the exclusion.

It should work. TM should start backing it up to whatever storage device your Mac is configured to use for TM backups. If you look at the TM backup device after the first backup has been made, you will see backup sets for two different storage devices.

If the Time Machine volume is APFS, this means that the root of each snapshot will have two (or more) directories - one for each volume it is backing up.

If the source volume is APFS (not recommended for a hard drive, for performance reasons), then it will also maintain 24 hours of hourly local backups.

I’m not sure, however, what you mean by “such that I could use with another machine when needed”.

Are you talking about transporting your TM backups to another computer? I guess you could do that, but I wouldn’t recommend it. I don’t think it’s a good idea to separate a TM volume from the computer that is using it, unless you are using it to restore data from it.

Or are you talking about ejecting your “Air Media” drive and connect it to the other computer. There’s no problem doing that. When you re-connect it back to the original computer, TM backups will resume.

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It should work just fine. I use this exact same kind of setup, with my media (images, music, and video) on an external drive, and TM backing up that volume, along with my MBP’s internal SSD, to a different external drive. It works like a charm.

One thing I would recommend for this is to get the largest drive you can afford for your TM drive. Media files can fill a lot of space very quickly without you realizing it. You don’t want this to interfere with TM’s ability to retain backups of your main drive.

Fair question. The other machine would be the one that I someday buy after I retire the current MacBook Air.

A other possible option is to create an alias to the external drive, say on your Mac desktop. I haven’t tried this for TM but the powerful Mac alias feature is sometimes useful for tricking apps to treat a folder/drive as local.
Also consider having more than one external drive for use with Time Machine. TM automatically backs up to available (mounted) drives. That way you have a backup of the backup!

OK. While you probably can restore that TM volume onto the new Mac, there’s probably no reason to do so since it’s an external drive containing documents. Just plug the drive into the new computer.

In order to get the new Mac to recognize it as your default photo and music libraries…

For Photos, you probably have one big package (that is, a folder pretending to be a file) called Photos Library or something similar. Just double-click it on the new computer and Photos should open it. I think it will be the default afterward. From within the Photos preferences, you can click a button to make it your default iCloud photo library. Once this is done, you can delete the pre-existing (probably empty) Photos library.

For iTunes, it will depend on how your library is currently set up. The master database is a file named iTunes Library.itl (or something similar). If this is on the external volume with your music, then you can just double-click it on the new Mac (which will, I think migrate the data to a Music database and then start using it as the default). If it’s on your Mac’s internal volume, with the music external, then Migration Assistant should copy it to the new Mac. If your external drive is connected when you open it, it should also just work - migrate the database from iTunes to Music and then start using it.

Similarly if you’re already using Music in your current system, but instead of being an iTunes library (a .itl file), it will be a file named Music Library or something similar.

For both apps, you can also hold Option while launching the app and it will give you a mechanism to select a library to open. You can pick the one you want and continue from there.

At least that’s how I expect it will work.