External Drive - Can't change Format from APFS to HFS+

[krs]

Looks as if I got myself into a dead end.
Started with a 2 TB USB 3 external HDD
Created 3 HFS+ partitions of 1 TB, 500 GB and 500 GB
Did a SD backup of one Mac running 10.14 to the 1 TB partition and a backup of another Mac running 10.11 to one of the 500 GB partitions, so far so good
Later installed macOS 10.14 (on an experimental basis to upgrade the Mac running 10.11) from a USB installer on the other 500 GB partition (which had been formatted as HFS+ initially.
That all worked fine as well.
When 10.14 was installed on that second 500 GB partition, the install changed the format to APFS - that's normal as well.

Now comes the problem - I want to use the 500 GB partition with the 10.14 install on it to make another backup of the Mac running 10.11.
To start with, when I connect the external drive with the two backups and the 10.14 install to the Mac running 10.11, none of the three partitions show up on the desktop or in Disk Utilities. That Mac is a 2017 MacBook Air running 10.11
Don't understand why, I would at laest expect the HFS+ partitions to mount.
When I then connect the external HDD to the 2012 Mini running 10.14, all three partitions show up on the desktop and in Disk Utility, but when I try to erase the 10.14 macOS on the 500 GB partition on the HDD and format it to HFS+ (from APFS) to hopefully get the HDD to mount on the 2017 MBa, HFS+ does not show in the format drop down meny - I only get four variants of APFS.

And if I check Disk Utility on the 2012 Min (with 10.14), I see the external Toshiba drive, a "Container disk4 and in the container the two HFS+ back ups and the 500 GB APFS partition which I was trying to erase and format as HFS+

First question -
Does the external drive not mount on the 2017 MBa because one of the partitions is APFS?
The light on the drive comes on, I can hear it spinning, but it doesn't mount or show in DU.

 

[ferrarr]

Which Mac did you partition the external drive with? 10.11 or 10.14?

It looks like the hardware is formatted as APFS since the 2 partitions are in the container. And that's why they can't be seen.

 

[MacInWin]

When 10.14 was installed on that second 500 GB partition, the install changed the format to APFS - that's normal as well.

Thee entire drive probably converted to APFS at that point, with your two HFS+ partitions somehow included now as Volumes in the Container and as Bob said above, now the HFS+ cannot be seen because they are inside the APFS Container.

Don't know how that happened, but it fits with the situation you are in.

The only "fix" I can think of is to mount the drive on a Mac that can see into the Container, then copy the two HFS+ Volume contents to a different location (new drive?, Internal storage?, Whatever?) and then repartition, erase, reformat the entire drive and start over.

And generally, I don't put more that one bootable system per drive. Too many ways it can go south on you.

 

[krs]

Which Mac did you partition the external drive with? 10.11 or 10.14?

10.14

It looks like the hardware is formatted as APFS since the 2 partitions are in the container. And that's why they can't be seen.

Yes, makes sense.
But is also a huge problem in a way.
When I did the SD backup for 10.11 and 10.14 on the two different HFS+ partitions, they both worked fine on their individual Macs when I tested them.
Then some time later I did the 10.14 install on the third partition and suddenly the backup becomes invisible (ie unusable) on the 10.11 Mac.
That shouldn't have happened IMHO

 

[krs]

Thee entire drive probably converted to APFS at that point, with your two HFS+ partitions somehow included now as Volumes in the Container and as Bob said above, now the HFS+ cannot be seen because they are inside the APFS Container.

Don't know how that happened, but it fits with the situation you are in.

The only "fix" I can think of is to mount the drive on a Mac that can see into the Container, then copy the two HFS+ Volume contents to a different location (new drive?, Internal storage?, Whatever?) and then repartition, erase, reformat the entire drive and start over.

Yes, that seems to be the only option. I'm going to order another external drive today, not enough room anywhere else.
I like to do leapfrog backups so I don't want to just erase what I have and do another backup which would be the other option.