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.
Best Answer
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).