When I updated my mid-2011 MacBook Air from Sierra to High Sierra last year I had some installation issues... which resulted in creating at least 2, maybe 3 disk images in my recovery drive.
I have disk image files where they don't belong (like keychain)and my system compensated by putting them in other locations.
The repercussions are greater than I can list here... The two that are most troublesome are:
/dev/disk4 in /Volumes and /dev/disk12 on /Volumes/untitled.
-bash-3.2# mount
/dev/disk1s1 on / (hfs, local, read-only)
devfs on /dev (devfs, local, nobrowse)
/dev/disk3 on /private/var/log (hfs, local, union, nobrowse)
/dev/disk4 on /Volumes (hfs, local, union, nobrowse)
/dev/disk5 on /private/var/tmp (hfs, local, union, nobrowse)
/dev/disk6 on /private/var/run (hfs, local, union, nobrowse)
/dev/disk7 on /private/tmp (hfs, local, union, nobrowse)
/dev/disk8 on /System/Installation (hfs, local, union, nobrowse)
/dev/disk9 on /private/var/db (hfs, local, union, nobrowse)
/dev/disk10 on /private/var/folders (hfs, local, union, nobrowse)
/dev/disk11 on /private/var/root/Library (hfs, local, union, nobrowse)
/dev/disk13 on /private/var/root/Library/Containers (hfs, local, union, nobrowse)
/dev/disk14 on /Library/Preferences (hfs, local, union, nobrowse)
/dev/disk15 on /Library/Preferences/SystemConfiguration (hfs, local, union, nobrowse)
/dev/disk16 on /Library/Keychains (hfs, local, union, nobrowse)
/dev/disk17 on /private/var/tmp/RecoveryTemp (hfs, local, union, nobrowse)
/dev/disk18 on /private/var/tmp/InstallerCookies (hfs, local, union, nobrowse)
/dev/disk2 on /Volumes/Macintosh HD (hfs, local, journaled)
/dev/disk12 on /Volumes/untitled (hfs, local, nodev, nosuid, no owners)
/dev/disk4 in /Volumesis normal but/dev/disk12 on /Volumes/untitledmay not be. I can't boot a High Sierra system till tomorrow to see if I have one as well. Looking on a macOS Catalina system it's not there. What are the issues you're having? – user3439894 Dec 06 '19 at 05:15