I have a 2012 Mac Mini with 250GB SSD, and an external USB drive used for data files. I run parallels but as far as I can tell the VM is actually on the built-in drive.

By accident I discovered that turning off the USB drive causes several apps including Chrome to instantly close; the Mac continues to run but comes to an ugly hung state with some apps still running, others unresponsive.

I also noticed if the USB drive is off/unplugged when I turn the Mac on, I get the "no drive" folder-with-question-mark hold screen.

Is this normal behaviour, I'd assume even an ugly device removal would not topple the system if it's just data files that are not in use at the time. I wonder if it indicates I have part of my Mac system files or core apps on the external drive. Is this likely or even possible - MacOS seems pretty strict on what you have to install on the primary/system drive.

I have tested further and discovered that if I turn the external drive off before booting the Mac it will not boot - I get the folder with a question mark in it screen. How do I figure out which system files are on the external drive and remedy this?

As requested:

>sudo systemsetup -liststartupdisks

/dev/disk1s2 on / (hfs, local, journaled)
devfs on /dev (devfs, local, nobrowse)
map -hosts on /net (autofs, nosuid, automounted, nobrowse)
map auto_home on /home (autofs, automounted, nobrowse)
/dev/disk0s2 on /Volumes/Data (hfs, local, journaled)

>diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Data                    999.9 GB   disk0s2

/dev/disk1 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *256.1 GB   disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                  Apple_HFS JDX_MX100_1             255.2 GB   disk1s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk1s3

I should say, this means absolutely nothing to me!

  • All you should get is a warning that the disk was not ejected properly. If certain apps shut down when you power off the drive then they may have files open on that drive. If you leave the drive off, reboot and then launch those apps again what happens? Feb 22 '18 at 18:41
  • Reboot and hold down the Option key. This should bring up all the bootable volumes. In macOS, you can also go to System Preferences > Startup Disk to see the disks that are bootable. Usually a question mark at boot means that the internal disk is damaged or somehow non-bootable.
    – IconDaemon
    Feb 22 '18 at 18:42
  • So if I turn the disk off before starting the Mac, it won't boot - I get the question mark in a folder screen. How do I figure out what I've done, and fix it?
    – Mr. Boy
    Feb 27 '18 at 15:29
  • @klanomath done, whatever this means!
    – Mr. Boy
    Feb 28 '18 at 22:43
  • 1
    The question as well as the answer is completely valid (except the last sentence > prank => selfprank) ;-)
    – klanomath
    Mar 2 '18 at 14:47

The command sudo systemsetup -liststartupdisks lists all bootable volumes and where to find the file boot.efi.

sudo systemsetup -liststartupdisks

The command mount lists all mounted volumes:

/dev/disk1s2 on / (hfs, local, journaled)
/dev/disk0s2 on /Volumes/Data (hfs, local, journaled)

The first and the second command show that you have one bootable volume at


on disk1s2.

If you'd have second bootable volume on disk0s2 the first command would show a second entry like:


The command diskutil list is similar to mount but shows all partitions of all attached drives and disk images as well as the state/kind of the drive (internal|external|disk_image|APFS/CS_container).

Since your external drive contains the only boot volume with all required system files and folders, turning it off means crashing your Mac. Turning the Mac on without the system drive attached (the external drive!) means no bootable system volume (= ...get the "no drive" folder-with-question-mark hold screen...).

Solution: simply swap the drives! (Victim of a prank?)

  • So yeah, I migrated the boot-partition to the external SSD drive because I didn't want to install an internal SSD. And then forgot until you made it clear what was wrong. I could move the partition back to the internal drive but then it would be HDD not SSD. You've solved the mystery, I need to write a new question about problems this is causing.
    – Mr. Boy
    Mar 2 '18 at 14:49

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