I use LM18 with 3 external hard drives (in addition to 2 internal ones), two of which always get recognized/mounted when I start Linux. The third one, however, doesn't (although Win7 always recognizes it).
Googling for a solution didn't really find anything helpful. Except to open the Terminal and run 'sudo blkid', which showed all hard drives, internal and external, partitions, etc. just not the elusive 3rd external HDD

Many thanks.
MintDude2013 aka John