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I'm trying to diagnose & fix some problem on my MacBook Pro - the machine randomly hard-crashes instantly & shuts down.

I have a somewhat-old Time Machine backup of the drive on an external USB disk, and I would like to update that backup. The problem is that it keeps crashing in the middle of backups, then forcing a complete disk scan, which takes a really long time, so it never gets to complete a new snapshot.

To try to make progress, I created an external bootable drive, which I can use to get the laptop started. I can also mount my Time Machine backup drive - both are external USB drives. I was hoping to tell Time Machine to resume backing up the laptop's internal drive to the Time Machine drive, but I don't see any obvious way to do that.

Is there some way I'm missing? Maybe using tmutil from the command line?

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  • A word of advice, if your MacBook Pro is older than around 2013, remove the SATA Disk or SSD, plug it into an external drive and connect that to another Mac. Use a block based copy application such as Carbon Copy Cloner to backup your HDD or SSD to a folder or better yet to another drive connected to the Mac. If your MacBook is newer than 2013, continue with the bootable Mac OS you made and install something like Carbon Copy Cloner to backup to your Time Machine external drive.
    – Tmanok
    Mar 1, 2019 at 2:51
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    @Tmanok That would make a clone of the drive, which isn't what I want to do - I want to resume the backup of my drive when it's not the backup drive. Whether that's the most productive thing to do is a separate question.
    – Ken Williams
    Mar 4, 2019 at 3:14
  • I edited the question to remove some of the "why" context, to make this just a question of whether a TM backup from a non-boot drive is possible.
    – Ken Williams
    Mar 4, 2019 at 3:20
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    Gotcha, apologies for my lack of understanding and thank you for your improvement on the question :)
    – Tmanok
    Mar 5, 2019 at 4:15

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Time Machine is optimized to create and maintain a history of file-level backups of all files/folders on your system. What you rather need is a disk/image level backup which just dumps the whole drive onto an external drive. You can afterwards either copy the whole image back again or, probably more useful, get specific folders back.

The two applications people tend to use for this are CCC and SuperDuper!. You also need an additional external USB drive at least as big as your internal drive.

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  • Well, that's not really what I'm asking - I want to create/resume a Time Machine backup of a drive that's not the startup drive.
    – Ken Williams
    Mar 4, 2019 at 3:07
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    @KenWilliams I‘m aware of that. But I wouldn‘t trust a TM backup any longer whose update was interrupted several times. I also wouldn’t trust a drive which has failed several times already. Also booting from another drive changes mount points/paths so I‘m not even sure TM is even able to pick up again. If you want to backup your data before you replace the main drive you are better of with a one-off image backup onto a new drive
    – nohillside ♦
    Mar 4, 2019 at 6:12
  • I definitely don't dispute that an image backup would be a more straightforward way to recovery, but I'm trying to ask a pretty narrow question I haven't seen addressed anywhere before.
    – Ken Williams
    Mar 5, 2019 at 16:34

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