On the same USB-3.0 bus (MB-Pro Catalina, 10.15.6), I always have a 1-td HDD and sometimes have a 2-tb HDD, which is split into 1-tb partitions.

Time Machine is set to back-up to both the 1-tb HDD and 1 of the 1-tb partitions.

::problem:: MacOS sometimes unmounts the 2-tb HDD drive when I put it to sleep. Sometimes I leave the 2-tb disconnected for a few days because the extra TM bkup and data on the 2nd 1-tb partition is not that important. But this makes TM complain. Yet I thought TM should seamlessly just continue to use the drive it can find if it can't find the second drive in a dual drive configuration? The constant warnings make me want to change TM to only use the one reliable 1-tb drive.

I'd really like for the 2-tb drive to stop unmounting. It is 5-years old, has been used extensively, and kept immobile on my desk.

  • I have two destinations as well. Time Machine alternates when both are present, and skip any non-present destination. Only reminder I see is after a substantial number of days, where I am reminded that a destination has not been available. I think it is something else you see. Commented Oct 10, 2020 at 13:47

2 Answers 2


I have Time Capsule and Time Machine setup on my MacBook Pro.

The Time Capsule is always available at home. The Time Machine is only available sometimes.

macOS never complains about the Time Machine drive being unavailable, unless it's been like 7 days (which almost never happens).

  • Thanks for confirming its supposed to work like I think. So, for the next few days, before putting my MB to sleep, I'll purposely unmount the disk with Disk Utility, rather than just let it crash-off, and see how it goes.
    – rppkgai
    Commented Oct 10, 2020 at 5:12
  • I read there is some daemon to run that might help with the unmounting that disk. I don't have the experience on this, but I'm thinking that replacing the old 2-tb HDD with an SSD would 100% solve the issue and let me focus on actual work.
    – rppkgai
    Commented Oct 10, 2020 at 8:53

Apparently I'm confused... I thought that Time Machine was the software part of the backup, and Time Capsule, or a properly-formatted external drive, was the hardware part of the backup.

At any rate, I use two separate drives for Time Machine backups: one is a partition on an old Time Capsule WiFi AP/HDD unit, the other is a small USB drive that lives on my desk & occupies one of the USB-C ports when connected. This is how it looks on my Macbook (Catalina). It alternates between the two drives, and (I assume) would alternate between other drives if I added them:

Time Machine GUI

I have not seen any system-initiated unmounts while my system is sleeping, but I don't ever select Sleep from the Apple menu item - I simply close the lid. I do keep the charger connected most of the time. When I'm using the MB away from my desk, it does its backups over the WiFi to the Time Capsule, and eventually it will complain about a missing backup drive. Likewise, when I'm on travel and use the USB drive, it will complain infrequently. In either case Time Machine will continue its backups to the available drive.

One thing that you should check is the Options... button at the bottom of the Time Machine window. Your situation may be different, but in my case, the USB-mounted drive is excluded from backups. This was confusing to me initially as I thought that the Time Capsule partition should also be excluded. However, when I added it to the list, the system removed it immediately. Why? Not sure, but perhaps the system is programmed to automatically prevent a Time Capsule from being backed up?

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