hard drive – I assigned 2 disks to TimeMachine and when 1 goes missing, TM complains, but it should seamlessly continue to use the 1 that is present?

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 on the 2-tb HDD.

::problem:: MacOS sometimes unmounts the 2-tb HDD drive when I put it to sleep. And this makes TM complain, and I thought TM should seamlessly just continue to use the 1 disk that is present if it can’t find the 2-tb disk???

I thought TimeMachine would be cool with having 2 disks assigned to it, and when 1 is missing, it just continues to backs-up to the 1 that is present. Then, when the 2nd disk shows-up again, it switches-over and seamlessly makes the next back-up to that re-attached disk??? For me, TM complains about “*Not able to make a back-up in X number of days.” which is a lie, because it is still backing-up to the 1 disk that is present.

My 2-tb HDD unmounts some nights, and sometimes not. And, that is fine. I wish TM would be quiet and not complain. Or, is there a real issue? Or, I’d really like for the 2-tb drive to stop unmounting.