Why does new Time Machine SSD show up on my iMac but not on my wife's?

[Alwyn]

Having recently bought an external SSD to use as a second Time Machine I have successfully done a back up from my 2017 iMac. When I tried to back up my wife's 2011 iMac I got a message saying that the SSD didn't have a recognisable format so would need to be erased.

As it's formatted as APFS I found this odd. I decided to partition the drive and set up a partitioned drive as Sarah's iMac TM2, see attachment from Finder.

In System Preferences I opened Time Machine, clicked on Add/Remove backup disks but it didn't appear at all, see attachment.

 

[Randy B. Singer]

...I tried to back up my wife's 2011 iMac I got a message saying that the SSD didn't have a recognisable format so would need to be erased.

As it's formatted as APFS I found this odd.

macOS prior to High Sierra (10.13) cannot read APFS drives. If you have any Macs running macOS prior to High Sierra, those Macs will not be able to read an external drive if it has been formatted APFS.

https://iboysoft.com/wiki/apfs.html

 

[Alwyn]

macOS prior to High Sierra (10.13) cannot read APFS drives. If you have any Macs running macOS prior to High Sierra, those Macs will not be able to read an external drive if it has been formatted APFS.

Apple File System (APFS): Is It The Best Format for A Drive

Thanks I realised that that was the problem before picking up your reply so I checked to see how her 2011 iMac was formatted; it's Mac OS Extended (Journaled). So, I erased the SSD formatting it to that format then partitioned the drive allocating 600GB to my Time Machine and 400GB to hers. When I went into System Settings/Time Machine/Add, both TMs appeared so I clicked on mine and it started the backup overnight. This morning there were still 4 hours to go but when I checked 2 hours later a message appeared to say that there was insufficient space to complete the backup.

But, as the attachment shows there appears to be plenty of space. When I click on my Time Machine/Info it shows that it's being backed up in APFS. I don't know if that is a factor. Her partition is still formatted as Mac OS Extended (Journaled).

My HDD Time Machine has worked fine for years backing up from both our iMacs although it doesn't appear to be partitioned. Its format is Mac OS Extended (Journaled).

is it possible that Time Machine is trying to back up internal HDD as well as the external SSD with which I started iMac?

 

[Rod]

If, as I suspect "it's Mac OS Extended (Journaled). So, I erased the SSD formatting it to that format" you formatted the entire SSD to Mac OS Extended (Journaled) before partitioning it then you would have had to use Disk Utility to reformat your partition to APFS before using it for TM.
To be honest, I'm not even sure that's possible. Given that you created the formatting on your (newer) Mac I would have been inclined to format it all as APFS first, partition it then use DU to reformat the partition for your wife to Mac OS Extended (Journaled). Again, I have to say I've never attempted to do this myself so I'm unsure if it's possible. In my example I have a similar SSD set up, partitioned for my wife and I when we are o/seas but both partitions are APFS.

 

[Alwyn]

RESOLVED
Thanks Rod,

I erased my partition on external SSD ™ to APFS, cloned the other external SSD to the internal HDD, restarted to the latter and removed the external SSD so that ™ wouldn't include it in the backup. I successfully backed up my wife's iMac first then started a backup of mine. It took the best part of 24 hours but was successful. Time taken goes back to the problem with an HDD loaded with Ventura formatted to APFS.

It would be interesting to know if the majority of users of earlier iMacs/laptops stick with their original OS after experiencing slowdowns from upgrades.

 

[MacInWin]

It would be interesting to know if the majority of users of earlier iMacs/laptops stick with their original OS after experiencing slowdowns from upgrades.

I seriously doubt it. The system gets slow right after a major upgrade because the drive indexing is running in the background to enable Spotlight (and other search functions). Usually that clears up in a couple of days, although it can take longer in some cases.

Another source of slowdown is that if a user moves from a really old system using HFS+ format to a newer one using APFS, the optimization of the APFS for SSD can cause older rotating drives to be much slower in performance. That factor is not "fixable" and can sometime lead folks to revert.

But I think that most folks move on, unless it becomes unbearable.