Why could turning off my external data store hang my Mac, and how do I fix it? 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:

&gt;sudo systemsetup -liststartupdisks
/System/Library/CoreServices

&gt;mount
/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)

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

sudo systemsetup -liststartupdisks
/System/Library/CoreServices


The command mount lists all mounted volumes:

mount
/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

/System/Library/CoreServices


on disk1s2.

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

/Volumes/Data/System/Library/CoreServices


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?)