Disk Utility "x Not Mounted" meaning?

Thanks for the suggestion! Here's what Terminal returned for Container disk3:

Richards-iMac:~ rich$ diskutil list internal

/dev/disk3 (synthesized):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      APFS Container Scheme -                      +41.0 GB    disk3

                                 Physical Store disk1s3

   1:                APFS Volume ⁨Preboot⁩                 20.5 KB    disk3s2

   2:                APFS Volume ⁨Recovery⁩                20.5 KB    disk3s3

   3:                APFS Volume ⁨VM⁩                      17.2 GB    disk3s4

I've never looked at this level of storage before. Wasn't aware that separate Preboot and Recovery volumes were required. This readout allowed me to google "APFS Volume VM" and get some idea of what's going on here. Thanks!

While chasing this down, I found that available Disk Utility documentation didn't go into this level of detail. Do you happen to know of any Apple literature that explains what's going on at this level of storage? I'm not a system admin, but I like to look under the hood from time to time – at least have some idea of whether my storage system is kosher.

Thanks for the info and links!

I'll append the full diskutil list internal report for my internal Fusion drive, but I should stress that this isn't a troubleshooting request. As far as I know, everything is working. I just wanted to learn more about what I was seeing in Disk Utility.

It's been a couple of years since I reviewed my data organization and backup scheme, and there have been some ad hoc decisions that need to be re-examined and some inconsistencies that need to be ironed out. I was just at the stage of collecting volume & device info and creating a data-flow chart in Illustrator when I noticed those "not mounted" volumes and snapshots and realized that I didn't know enough to pass on whether the system is healthy. (Unidentified, unreadable system parts make me nervous.) From what I've seen and read so far, it doesn't appear that anything is amiss, but I want to take the opportunity brush up on AFPS before I start scripting in ChronoSync.

Very much appreciate your help and the links you sent!

Here's the readout from the diskutil list request:

/dev/disk0 (internal, physical):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        *121.3 GB   disk0

   1:                        EFI ⁨EFI⁩                     209.7 MB   disk0s1

   2:                 Apple_APFS ⁨Container disk2⁩         121.1 GB   disk0s2

/dev/disk1 (internal, physical):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        *2.0 TB     disk1

   1:                        EFI ⁨EFI⁩                     209.7 MB   disk1s1

   2:                 Apple_APFS ⁨Container disk2⁩         2.0 TB     disk1s2

   3:                 Apple_APFS ⁨Container disk3⁩         41.0 GB    disk1s3

/dev/disk2 (synthesized):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      APFS Container Scheme -                      +2.1 TB     disk2

                                 Physical Stores disk0s2, disk1s2

   1:                APFS Volume ⁨RHiMac - Data⁩           503.8 GB   disk2s1

   2:                APFS Volume ⁨Preboot⁩                 361.3 MB   disk2s2

   3:                APFS Volume ⁨Recovery⁩                1.1 GB     disk2s3

   4:                APFS Volume ⁨VM⁩                      1.1 MB     disk2s4

   5:                APFS Volume ⁨CEM⁩                     100.0 GB   disk2s5

   6:                APFS Volume ⁨BRB⁩                     169.7 GB   disk2s6

   7:                APFS Volume ⁨RHiMac⁩                  15.2 GB    disk2s7

   8:              APFS Snapshot ⁨com.apple.os.update-...⁩ 15.2 GB    disk2s7s1

/dev/disk3 (synthesized):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      APFS Container Scheme -                      +41.0 GB    disk3

                                 Physical Store disk1s3

   1:                APFS Volume ⁨Preboot⁩                 20.5 KB    disk3s2

   2:                APFS Volume ⁨Recovery⁩                20.5 KB    disk3s3

   3:                APFS Volume ⁨VM⁩                      17.2 GB    disk3s4

Richards-iMac:~ rich$ 

It looks like a remnant of installing a second OS on the drive.

Are you referring to /dev/disk3 (synthesized)? If so, then yes, that is quite likely. 41 GB looks like about the size of a volume I would create to install a new OS so that I could test it before upgrading globally. I now do my new OS testing on another machine, but this used to be our lead machine and would have been a likely subject for a test-OS drive install.

This machine is quite old (late 2015) and has been hanging around, waiting to be replaced by an Apple-silicon 27" iMac, if such a thing ever arrives. It's got a lot of history. It's also got enough space so that 41 GB staying out of service would not affect its performance. Or perhaps the remnant has never been required by the OS, but would be available if needed. ??