"How Do I Create Multiple MacOS Volumes or Partitions on an SSD or Hard Drive with APFS?"
Generally, when you have an SSD or hard drive formatted with the new Apple File System (APFS), you only want a single partition on it. You can then create multiple "volumes" within that partition. These volumes act just like partitions, except they share a single partition's space among themselves as needed. This avoids the kind of wasted space you always get with multiple drive partitions and their rigid size allocations.
The problem comes when you want to place more than one copy of a recent MacOS version on the drive -- say, both High Sierra and Mojave, or even duplicates of either one for testing or easy backup. An APFS volume with MacOS installed is accompanied in that same partition by three hidden, associated volumes, including the Recovery volume. Place another MacOS volume in that partition, and your system and utilities can become confused.
So, in this case, you do have to partition the drive instead of adding volumes. But on trying to, you may run into quirks and bugs in MacOS and Disk Utility, and possibly in your cloning app.
I'm no expert on this, but after trying several schemes and finally succeeding, I'll tell you the way things look to me. Keep in mind, though, that some conclusions may apply only to my own hardware and software setup. I was using Disk Utility and SuperDuper in High Sierra on a 2010 Mac Pro, and one of the SSDs was years old, branded by Apple itself.
-- If an APFS partition has multiple volumes, you can't use Disk Utility functions on ANY of them unless you're booted from a volume in that partition. If you're not, Disk Utility can't unmount them. UPDATE: You can resolve this by first using Disk Utility to MANUALLY unmount the other volumes in the partition -- and afterwards, to manually remount them.
-- Because of the hidden volumes, you cannot reliably place a second MacOS volume within an APFS partition. Though you can boot from either one, this caused a freeze for me when running Disk Utility First Aid on the second one.
-- Again because of the hidden volumes, you cannot reliably create a second APFS partition by splitting one that already hosts a MacOS volume. This too caused a freeze during Disk Utility First Aid.
-- You cannot copy MacOS between one volume and another in the same partition if it contains the boot volume, because the Recovery volume is locked. (Thanks to Dave Ninian of Shirt Pocket for the explanation.)
-- At least in some cases, APFS partitions have minimum sizes -- a fact I have not seen mentioned anywhere. A partition with a MacOS volume must be at least 240GB, even in the absense of backup snapshots. How this minimum is affected by other types and numbers of volumes in the partition, I haven't tested.
So, after all that, what CAN you count on doing? I believe the most reliable plan is to set up the drive with one APFS partition for each MacOS copy you'll need, and to do it BEFORE you clone MacOS into any of them. Make each partition at least 240GB.
And if the drive already hosts a MacOS volume? Boot into that volume, then try to partition the disk with one or more new, NON-APFS volumes. For instance, for Format on the new partitions, tell Disk Utility to use MacOS Extended (Journaled), the format of Apple's old file system, HFS+. Then once those partitions are created, use Disk Utility again to erase each of them with the APFS format instead.
Either of these methods will create APFS partitions with NO hidden volumes to start with. Then, when you clone MacOS into them, the hidden volumes will be created correctly, and you should have no trouble at all.
Update, Sept. 1, 2019 -- Cloning MacOS into an APFS partition does NOT create the hidden volumes. You have to install MacOS on it with an Apple installer. This can be done before or after you clone your system to that partition.
Also, having two MacOS partitions on your SSD can confuse the latest High Sierra Security Update installer, as of 2019-004, causing it to fail.
The problem comes when you want to place more than one copy of a recent MacOS version on the drive -- say, both High Sierra and Mojave, or even duplicates of either one for testing or easy backup. An APFS volume with MacOS installed is accompanied in that same partition by three hidden, associated volumes, including the Recovery volume. Place another MacOS volume in that partition, and your system and utilities can become confused.
So, in this case, you do have to partition the drive instead of adding volumes. But on trying to, you may run into quirks and bugs in MacOS and Disk Utility, and possibly in your cloning app.
I'm no expert on this, but after trying several schemes and finally succeeding, I'll tell you the way things look to me. Keep in mind, though, that some conclusions may apply only to my own hardware and software setup. I was using Disk Utility and SuperDuper in High Sierra on a 2010 Mac Pro, and one of the SSDs was years old, branded by Apple itself.
-- If an APFS partition has multiple volumes, you can't use Disk Utility functions on ANY of them unless you're booted from a volume in that partition. If you're not, Disk Utility can't unmount them. UPDATE: You can resolve this by first using Disk Utility to MANUALLY unmount the other volumes in the partition -- and afterwards, to manually remount them.
-- Because of the hidden volumes, you cannot reliably place a second MacOS volume within an APFS partition. Though you can boot from either one, this caused a freeze for me when running Disk Utility First Aid on the second one.
-- Again because of the hidden volumes, you cannot reliably create a second APFS partition by splitting one that already hosts a MacOS volume. This too caused a freeze during Disk Utility First Aid.
-- You cannot copy MacOS between one volume and another in the same partition if it contains the boot volume, because the Recovery volume is locked. (Thanks to Dave Ninian of Shirt Pocket for the explanation.)
-- At least in some cases, APFS partitions have minimum sizes -- a fact I have not seen mentioned anywhere. A partition with a MacOS volume must be at least 240GB, even in the absense of backup snapshots. How this minimum is affected by other types and numbers of volumes in the partition, I haven't tested.
And if the drive already hosts a MacOS volume? Boot into that volume, then try to partition the disk with one or more new, NON-APFS volumes. For instance, for Format on the new partitions, tell Disk Utility to use MacOS Extended (Journaled), the format of Apple's old file system, HFS+. Then once those partitions are created, use Disk Utility again to erase each of them with the APFS format instead.
Either of these methods will create APFS partitions with NO hidden volumes to start with. Then, when you clone MacOS into them, the hidden volumes will be created correctly, and you should have no trouble at all.
Update, Sept. 1, 2019 -- Cloning MacOS into an APFS partition does NOT create the hidden volumes. You have to install MacOS on it with an Apple installer. This can be done before or after you clone your system to that partition.
Also, having two MacOS partitions on your SSD can confuse the latest High Sierra Security Update installer, as of 2019-004, causing it to fail.
Update, Oct. 3, 2023 -- Mounting an APFS partition is a lot more involved for the Mac than mounting HFS+. On an old Mac like mine, having several APFS partitions, even on SSD, apparently can cause the Finder to become unresponsive for a minute or so after a cold startup.
To stop this, I wound up wiping one of my SSDs clean and installing several HFS+ partitions for my extra copies of High Sierra. Yes, High Sierra can run on an HFS+ SSD -- you just can't install it on one! The solution is to instead clone it to the partition, and also make sure you have TRIM enabled on your Mac to keep your SSD efficient. Just be aware that you won't have a Recovery volume on this drive.
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