Using APFS Time Machine Backup with Big Sur

I have had two physical drives formatted in Mac OS Extended (HFS+). Both of those have two partitions: one TimeMachine partition and one partition for files (not TM). They work fine. TM backs up to both of those two partitions that are located on two different drives.

I got a new drive (a third physical drive) and since I am now on Big Sur, I decided to try APFS and store Time Machine on one partition and misc files on the other partition.

At first, I created one 'container' and created two APFS System Volumes (sorry if the terminology is not exactly correct). When I went into Time Machine, I could not find the Volume I wanted to use for TM backup, nor any volume from that drive.

On reading some comments, I figured I might need a partition of a defined size (rather than the APFS volume which can change size. I think?). So I partitioned the hard drive into two APFS 'containers' of a fixed size, each one half the size of the drive. The Time Machine is on the second 'container' since the first container has files I had already backed up to this drive.

Now, suddenly, TM recognizes the second partition. It did not do so earlier today. Go figure.

So I am good to go!

Thanks.

I try to avoid using the command line, but I will try it.

I created the partition using Disk Utility. It works for disks formatted in 'Mac OS Extended (Journaled)'. I have two TimeMachine disks, each partitioned in half and those are recognized and work as backups.

But in this disk, I have two separate 'Container's' of a fixed size. The second one is the one I had hoped to use for a TimeMachine backup. That appears on my list of drives, but does not show up as an option in TimeMachine preferences.

"I would use a drive dedicated to TM..."

I have not had to use a separate drive with the older Apple format. I portioned each of those in half. Half is TM and half is for other files.

The advantage of doing that is if one drive fails, I still have a TM backup drive that I can access.

I think we might be misunderstanding each other.

I have, physically, two drives. I want to partition each one. And use those two drives as a backup. I am not partitioning one drive and expecting to have two TM machines backups on one. I realize that would be silly.

If I take one of my two drives and make it a dedicated TM backup, that would work, but all my TimeMachine backup is on one drive. If that fails, I lose my TM backup.

But what I want to do is take my two physical drives and partition them. That way I have two TM backups. And if one fails, I still have the other. I can do that with the older Mac format. But I have been unable to partition and use the second physical drive partitioned in APFS.