Time Machine Drive Will Not Mount and I Cannot Erase It

[wncmacs]

First off, this issue is with a spinning hard disk that I partitioned into a storage side and a Time Machine backup side using APFS.

My Time Machine backup disk has failed to mount. I opened Disk First Aid and try to mount the drive which fails. I then run Disk First Aid and get the following:

So I thought I would just erase the disk and start afresh with Time Machine backups. When I tried that, I get this message:

The other disk partition of the drive is mounting just fine...only the Time Machine portion is failing to mount and I can't seem to get it to erase.

Any suggestions on what I should try to remedy this, or is the Time Machine partition just hosed?

 

[Fishrrman]

My advice (and you may not like it).

First, copy everything from "the data partition" to ANOTHER DRIVE.
(or else this data is going to be lost)

Next, open disk utility.
Important: go to the view menu and choose "show all devices".

Now, click on the "line on the left" that represents the ENTIRE EXTERNAL DRIVE -- not any individual partition (such as the tm partition).

NOW click the erase button and look at what choices you're offered.

If you can erase the ENTIRE drive to APFS, do so.
If du won't let you do that, does it let you erase the drive to either MS-DOS or exFAT?

IF you can do that, DO SO.

Once that's done, click the erase button AGAIN.
See if THIS TIME it will let you erase the drive to APFS.

Does this work?
The objective here is to get the drive erased.

More advice (that you're not going to like):

STOP USING time machine for backups.
You've seen how well it worked for you.

START USING either CarbonCopyCloner or SuperDuper.

CCC (besides creating clones) has the ability to create a "safety net" so that previous versions of files aren't deleted when the cloned backup is updated.

Final advice:
Don't partition a drive that's intended to be used for backups.
Let the entire drive "be the backup drive".
Things will just go easier that way in the future...