macOS Startup Disk


MacOS 10.14.6, iMac16,2 (Late 2015)

I have cloned my Fusion drive to a Samsung T5 SSD (1TB) using SuperDuper. When I open System Preferences > Startup Disk other bootable clones on HDs show up in a list of possible startup volumes, but the T5 does not show up. On the other hand, if I startup with Option held down, the T5 does show up and I can boot from it.

Does anyone here have any idea what’s going on?


OP here. When I tried to install BigSur on the T5 the installer told me it had the wrong partition scheme (wasn’t GUID). I’ve erased and reformatted the T5 with GUID and am cloning again with SuperDuper.

It’s still odd to me that I could boot from the T5 (non-GUID) when holding Option on restart but it didn’t show up in System Preferences > Startup Disk.

It’s been the case for some time that even though you may be able to boot a Mac from an APM-partitioned drive, Apple won’t let you install to one.

I think (but am not 100% sure) this goes all the way back to the dawn of the Intel Macs and macOS 10.5.

Some Big Sur startup volumes don’t appear in the Startup Disk Preference Pane

Back again to continue the saga. My goal was to be able to experiment and see how Big Sur might affect the rest of my setup so wanted to install it on the clone not as a fresh installation. [I probably should start anew with a fresh install as cruft has been accumulating since the early 90’s. I’ve recently discovered some AppleWorks documents that came over from a IIgs!]

I assumed the Big Sur installer would convert the target disk to APFS as I think the Mojave and/or Catalina installers did. (I’ve skipped Catastrophelina). But, the Big Sur installer told me the file system had to already be APFS. Bummer. Erase, reformat, reclone, many hours later I was finally up and running with Big Sur on my system clone. WOW that SSD is so much faster than my Fusion drive. Now I’m hoping I can hold out until the Mx shows up in an iMac!!

As mentioned by Ryoichi with reference to the bombich info, Big Sur does not appear in the Startup Disk Preference Pane when the machine is booted into Mojave on the internal Fusion drive, but can be selected in the Startup Manager by holding the Option key on reboot.

But I am still confused and hope those here who are heavily versed in APFS, partition maps, and the secret sauce that is macOS 11 will be able to inform me. I intended to name the T5 Big Sur but Disk Utilities shows that the Big Sur installer kept the device name ‘Samsung Portable SSD T5 Media’ and created a ‘Container disk6’ containing something named ‘Big Sur - Data’ and something else named ‘Update’ which appear to be two different logical references to the same physical T5 as they both have the same capacity, i.e. they are not two separate partitions as I understand it. Only ‘Big Sur - Data’ shows up in the Locations list in the Mojave Finder.

So, can anyone explain (or point me to an explanation of) what the macOS 11 installer has done to the startup disk?

In Disk Utility, you will see a three-tiered hierarchy for disks with APFS volumes:

  • At the top level is the physical disk. This name is derived from the drive’s internal ID and is not (as far as I know) modifiable. On my system, my internal SSD (formatted APFS) is named “APPLE SSD AP2048M Media” and an external USB hard drive (formatted HFS+) is named “TOSHIBA HDWQ140 Media”.
  • Below that are the names of the actual disk partitions. A partition containing an HFS+ volume (or any volume other than APFS, as far as I know) is 100% used by that volume. For APFS, however, the partition contains a “container”. The container may contain multiple volumes and actually gets its own disk device name to simplify this.
  • Within an APFS container are volumes. Each volume is a separate file system, but all volumes within a single container share the same free space(!).
    • For a data disk, your APFS container may have only one volume.
    • For a system disk, it will have five standard volumes. The system volume (which is not writable under normal cirumstances), a data volume (where everything else is stored), and three special hidden volumes: Preboot, Recovery and VM.

You can see them all best from the CLI diskutil command. On my mac (running macOS 10.15, Catalina), I see:

> diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         2.0 TB     disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +2.0 TB     disk1
                                 Physical Store disk0s2
   1:                APFS Volume Balrog                  11.2 GB    disk1s1
   2:                APFS Volume Balrog - Data           1.0 TB     disk1s2
   3:                APFS Volume Preboot                 85.7 MB    disk1s3
   4:                APFS Volume Recovery                529.0 MB   disk1s4
   5:                APFS Volume VM                      2.1 GB     disk1s5

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Time Machine            4.0 TB     disk2s2

The above output shows:

  • /dev/disk0 is the first physical disk, internal to the computer. It is partitioned using the GUID partition scheme and contains two partitions:

    • EFI doesn’t do much on Macs (as far as I know). The EFI partition is critical to the boot process on computers with EFI firmware running other operating systems (for device drivers needed before the boot-loader loads), but I don’t think Apple uses it in normal operation (but it may be used for temporary storage during firmware updates)
    • Container disk1 is an APFS container, which contains the entirity of the disk1 device.
  • /dev/disk1 is a “synthesized” disk, created entirely from the APFS container mentioned above. It contains five volumes (all APFS formatted, of course):

    • Balrog is the name of my boot disk and the name of my computer. It is the (normally read-only) system volume.
      • On macOS 10.13 (High Sierra) and 10.14 (Mojave), this is the entire volume and it is read/write.
    • Balrog - Data is the name of the data partition containing all the rest of my stuff. It only exists on macOS 10.15 (Catalina) and later.
      • macOS combines the (read-only) system volume with the data volume to produce the disk you see on your desktop.
      • macOS 11 (Big Sur) takes this a step further. Instead of combining the system volume with the data volume, it creates a read-only snapshot of the system volume and digitally signs (“seals”) that snapshot in order to detect any tampering. It combines the system sealed snapshot with your data volume to produce the disk you see on your desktop.
    • I’m not sure what Preboot is for, but the name strongly implies that it contains boot-loader code or other files needed before the OS kernel is fully up and running.
    • Recovery is your recovery volume. It’s what boots if you hold CMD-R when booting.
    • VM contains virtual memory caches
  • /dev/disk2 is my external (USB) physical disk. It is also partitioned using the GUID partition and contains two partitions:

    • EFI is the same as the EFI partition on the internal disk.
    • Time Machine is an HFS+ volume which I am using for Time Machine backups.

Within an APFS container, the different partitions have different “roles” defining what they’re meant for. You can see this using the diskutil apfs list command:

> diskutil apfs list
APFS Container (1 found)
+-- Container disk1 5769D378-EACA-42FF-88FC-ADF6543A8B60
    APFS Container Reference:     disk1
    Size (Capacity Ceiling):      2000796545024 B (2.0 TB)
    Capacity In Use By Volumes:   1014599278592 B (1.0 TB) (50.7% used)
    Capacity Not Allocated:       986197266432 B (986.2 GB) (49.3% free)
    +-< Physical Store disk0s2 69AAB1FC-BB26-4BDF-AE23-E3DE5C7CC66D
    |   -----------------------------------------------------------
    |   APFS Physical Store Disk:   disk0s2
    |   Size:                       2000796545024 B (2.0 TB)
    +-> Volume disk1s1 4466AEE6-6217-4854-9AB6-244DC4462E33
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk1s1 (System)
    |   Name:                      Balrog (Case-insensitive)
    |   Mount Point:               /
    |   Capacity Consumed:         11212120064 B (11.2 GB)
    |   FileVault:                 No (Encrypted at rest)
    +-> Volume disk1s2 6D9C2436-C56D-49B6-9D23-AC2119E83BE2
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk1s2 (Data)
    |   Name:                      Balrog - Data (Case-insensitive)
    |   Mount Point:               /System/Volumes/Data
    |   Capacity Consumed:         1000324194304 B (1.0 TB)
    |   FileVault:                 No (Encrypted at rest)
    +-> Volume disk1s3 A7380878-2A5C-4D61-914F-EF244E0933FB
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk1s3 (Preboot)
    |   Name:                      Preboot (Case-insensitive)
    |   Mount Point:               Not Mounted
    |   Capacity Consumed:         85733376 B (85.7 MB)
    |   FileVault:                 No
    +-> Volume disk1s4 6DB0EDBE-D188-4903-BDA6-532FEFB1C107
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk1s4 (Recovery)
    |   Name:                      Recovery (Case-insensitive)
    |   Mount Point:               Not Mounted
    |   Capacity Consumed:         528969728 B (529.0 MB)
    |   FileVault:                 No
    +-> Volume disk1s5 CEDAE56A-EDDC-46F5-BE30-27D62B2C683A
        APFS Volume Disk (Role):   disk1s5 (VM)
        Name:                      VM (Case-insensitive)
        Mount Point:               /private/var/vm
        Capacity Consumed:         2147504128 B (2.1 GB)
        FileVault:                 No (Encrypted at rest)

Your startup disk should look very similar, with at least five volumes in the container. One in the System role, one in the Data role, one Preboot, one Recovery and one VM.

I’m not sure what roles your volumes are, but I would assume that your “Update” volume is your system volume (probably named when the APFS container was created by the Big Sur installer) and “Big Sur - Data” is your data volume (named by you when you renamed the volume to “Big Sur”).

You should be able to rename the system volume, but I don’t think it matters, since that name will only be visible to disk utilities, not in the Finder.

For more information, see:

And check out the manual page for the command-line diskutil (man diskutil). Scroll down to the APFS section - there’s a lot of good information in there.

1 Like

Very helpful. Thanks for taking the time.

I echo @jaclay. Thank you.

A few more comments that may be useful here.

In my example, the APFS container is created from one partition on one physical volume (the internal SSD), but containers can actually be created from multiple physical volumes. Apple uses this to implement Fusion drives. It looks like you could also use it as a disk-spanning system a-la RAID-0, although I suspect actual RAID software would work better.

The man page also points out that APFS physical stores are not necessarily a single partition on a physical device. They may be data stores created from multiple physical devices (e.g. an AppleRAID Set disk).

Another key point to understanding APFS is that two or more volumes sharing a single container may be combined into a volume group. macOS uses volume groups to associate a volumes that are functionally linked but have to be kept separate for other reasons - like a bootable system’s “system” and “data” volumes. You can see available volume groups using the diskutil apfs listVolumeGroups command:

> diskutil apfs listVolumeGroups
APFS Container (1 found)
+-- Container disk1 5769D378-EACA-42FF-88FC-ADF6543A8B60
    +-> Volume Group 6D9C2436-C56D-49B6-9D23-AC2119E83BE2
        APFS Volume Disk (Role):   disk1s1 (System)
        Name:                      Balrog
        Volume UUID:               4466AEE6-6217-4854-9AB6-244DC4462E33
        Capacity Consumed:         11209293824 B (11.2 GB)
        APFS Volume Disk (Role):   disk1s2 (Data)
        Name:                      Balrog - Data
        Volume UUID:               6D9C2436-C56D-49B6-9D23-AC2119E83BE2
        Capacity Consumed:         1009791848448 B (1.0 TB)

In my case, you can see the system and data volumes sharing a group together.

1 Like

THANK YOU @Shamino!!! Your explanation has helped clear up a number of questions I had about what i was seeing in Disk Utility. I’m OK with Hebrew, Greek and bit of English but computer is still strange to me lol. Very helpful post.

This might be only tangentially related to the thread, but since upgrading to Big Sur, my original boot drive seems to be located in the Volumes folder of the System Folder. The drive that appears on my desktop doesn’t have my normal folder structure, though it has the same disk name.

I have to bring up the “hidden” disk’s folders to save files where I normally would. Though I have read through this thread and some others, the new file system’s workings are still murky to me. Any advice?