I've had this problem before once and recently it happened again.
When I start the computer it hangs during the Mint load-up screen with the message:
Key: Disk not mounted. Press S to skip or M to manually mount (or something like this anyway)
Sometimes I use software like PhotoRec and GParted to check friends damaged hard drives and recover their files for them. I use my external Hard disk Caddy to do this. I back-up the drive first and then start messing with it sometimes booting to a windows partition to use CHKDSK the windows disk tool along with other things such as Hyrons boot disk etc.
Somehow I seem to on occasion reboot to Linux Mint without removing the damaged drive, I think I've done it when I have been tired or frustrated but the end result is Mint automatically adds the drive to my start up. This is probably the Automount programme (I need to read up more on that one).
Result, on a boot up the computer expects to find a disk that is not there after I have removed it from the caddy.
To fix this, once booted into Mint and after the message (Press S to skip the mounting) I need to go into Terminal and change the fstab script which controls the disks expected to be found.
Terminal
sudo fdisk -l
As Root. This lists the drives. Check the ones available. sda (first physical disc), sda1 (first physical disc, partition 1), sdb2 (second physical disk, partition 2) for examples.
Terminal
gksudo gedit /etc/fstab
As Root. Use "gedit" text editor to view contents of file "fstab" located in folder "etc".
Once this is up and viewable I can see the instructions and options for each partition to be mounted. There is a description above each instruction with a hash (#) symbol in front of it which is used like the old REM comment tag in Basic's.
Anyway I use this hash (#) tag to comment out the instruction which loads the now no longer existent drive. I save the file to make the changes and use menu to reboot the machine.
The computer now loads as normal without searching for a non-existent drive.