Can an external hard drive in APFS format be partitioned for Time Machine and external storage

Yes, it can be done. But why do you want to convert the external drive to APFS in the first place?

APFS is optimized for SSDs, not HDDs. It is required only for your system drive (Mojave & later); and even in that case you could partition the physical drive into an APFS partition (for the system) and an HFS+ partition (for other uses). If this external drive is not your system drive, it is not necessary to convert it to APFS.

That said, if your external drive is an SSD perhaps it makes some sense to format it as APFS due to the way APFS manages file storage on SSDs. However if your external drive is an HDD I would advise keeping it as HFS+.

Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Notation - all methods of Backups or Clones are to dedicated single purposed External Drives.

I just did this: in using a Crucial 1Tb SSD over USB-C; I have Catalina.

• created a partition 550 Gb, formatted that partition as Macos Extended etc. An APFS partion sometimes will be reformatted to Macos ext by TM - in High Sierra for instance.
• named it TM on Crucial SSD
• opened TM; clicked on 'select drive', and then it shows up in the list (if you create a volume it wont show up . . .). Select that.
• Then it started its back-up process by itself

the other partition = 450 tB can be used as normal storage. I will use it as a preparation for Monterey, as an external boot.

Having two different backups on the same drive is a very bad idea - all drives fail sooner or later. So, when it does, you will lose both your backups.

Get another hard drive for the second backup - that will also take care of the formatting. You can format one for APFS and the other for Mac OS Extended.

Caution using a single drive for Multi-Purposes regardless of Partitioning or format.

ALL the Eggs in one basket and drive fails - which they do - presto - everything is gone. Data Recovery is expensive. The degree of success, especially on SSD is marginal, if at all.

In 1990, my employer sold a 600 MB SCSI storage device for $4500. Today, storage is far more reasonable, and sufficiently affordable that one should spend the money for a dedicated, suitably sized Time Machine drive and an additional drive for just external storage. That is what I do.

DCarlson93 wrote:

You're correct about this. After doing some research I found that APFS is better suited for SSD Time Machine Backups. HFS+/Extended Journal is better suited for an HDD Time Machine backup.

APFS can be used for used on an HDD Time Machine but it's considerably slower than Extended Journal because APFS is optimized for SSD. Welp, that just saved me a bunch of time and headache of wiping out my hard drives and reformatting them twice. Thank you.

For a backup drive, the optimizations or lack there of are not particularly important. The most significant issue with APFS on a spinning hard drive is fragmentation. APFS doesn’t really care because it is a non-issue on SSD. For a backup drive, files are not being read into memory then rewritten to disk. Fragmentation is almost nonexistent. Files are written once contiguously.

And, if creating a new TM backup on Big Sur, it will always be APFS.

You have to partition it prior to setting it up as a backup drive.

Time Machine will back up external drives, but I don’t know if it will back up that partition.

By the way, a TM partition cannot be used for other access . . . But that is why I made a separate partition . . .

Of course it can be done, but that does not mean it is a good idea.

Actually, it is a really bad idea. When your drive fails, you lose all your backups.