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I want to backup an external HD (A) that is only occasionally connected to another external HD (B).

I would like to use time machine for this, but it seems this isn't possible in my scenario.

Already using time machine to create backups of my Mac's internal HD (C) to yet another external HD (D), I can't setup backup jobs with different source and destination (A backups to B and C backups to D).

While looking for easy alternatives, I found it's possible to clone drives using disk utility. While not particularly elegant, since HD A is only occasionally connected, this could be good enough.

Still I am having two questions:

a) Is the error maybe on my side and I overlooked a setting in time machine? b) Will the cloning do any harm to my drive as it's not incremental, but basically reformats the drive every time?

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  • When you say that you connect drive A to B, I assume that you mean you are manually backing up the contents of A to B, and are trying to find a way to automate this process? If so, it's going to be really difficult to use Time Machine for this. You can use multiple drives as backup drives in Time Machine, and it will alternate between them each time it backs up, but I don't think you can use Time Machine to handle two wholly distinct, separate backups.
    – Fried Waffle
    Commented Dec 25, 2017 at 20:56
  • That is exactly what I was asking and part of an answer, thank you! I am just not able to word it that well, sorry! To do some kind of automation, I am currently using clone function in disk utility, but I am not sure if this is made for that job. @FriedWaffle
    – Sven
    Commented Dec 25, 2017 at 21:02

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As best I understand your question, you want to back up one external drive to another external drive, preferably using some sort of utility to automate it.

Time Machine is great for backing up a MacBook or iMac, and you can even set it up to alternate between multiple disks for redundancy, but Time Machine is not able to handle multiple distinct, separate backups on its own. It is designed to back up the system--and optionally any external drives that are formatted as either HFS+ or APFS--to its backup drive(s). It is not able to back up drive A to drive B while also backing up drive C to drive D. (At least not without a fair bit of fanangling each time you want to shift focus.)

As for cloning the drive via Disk Utility, it depends on what kind of drive you are cloning to. If you are using a hard disk drive (with a magnetic platter), there should be no unusual wear and tear. If you are using a solid state drive, it will wear out the cells faster as everything is being rewritten with each clone, rather than a few cells here and there.

That said, there are many paid options available for backing up data, usually ranging from $20-$50. It may be worth your time to just pick up a license for one of them.

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