Time Machine set up changing drive from Mac OS Extended (Journaled) to APFS, Ventura

Time Machine formats its drive APFS Case-sensitive. That doesn't cause the read-only problem.

If you create another volume on the drive, it won't be read-only. There is a note in the middle of the following guide with a link on how to create another Volume.

Types of disks you can use with Time Machine on Mac - Apple Support

You can also partition the drive such that the Time Machine container has a fixed size. You can also format the other container however you'd like.

Partition a physical disk in Disk Utility on Mac - Apple Support

You can only partition the device.

Either way a failure is a failure. One partition won’t be saved any more than a volume.

With a partition, you have to decide what size you may need, but you would be able to use a different file system.

Make sure you back it up.

LaneyHess wrote:

That's good to know—however I'm curious what problems it could cause. I have been using an external HD to backup my Mac as well as a holding place for other random files for decades and have not been aware of these problems...

Then you were probably using a large-enough hard drive for Time Machine. If you add too many non-Time Machine files, it can get confused when it is doing its calculations and delete too many old backups or simply refuse to make any new backups. When Time Machine does eventually flake out, it's very easy to reset. Just erase the hard drive and you're good to go in minutes. But if you have stored a lot of additional files on the drive, then you have to move those files first. And since you were probably doing this to avoid buying an archive drive, you probably don't have any place to put the files. So now you don't have a backup.

But I'm pretty sure this has all been solved with modern Time Machine.

Anyway, you don't want to partition the Time Machine drive. Just buy a new drive for archives. They're cheap. When you have multiple drives for archives and backups, your data is more secure because there is much less risk of multiple failures. But when you have everything on one drive, that's a single point of failure. If it dies, it takes everything. Did I mention you can also have multiple Time Machine backup drives?

The instructions are in the article O linked. But, select Show All Devices from the View pop up menu.

The device is the top level with the name and size in it. But, you can only partition the device. You can’t sub-partition a partition or volume.

That's how it works, now.

Partition the drive or create a Time Machine volume on the APFS formatted drive.

LaneyHess wrote:

Therefore the drive would be useless except to use solely for TM backups.

That’s a good thing. Trying to use a Time Machine drive for other things will cause problems.