It looks like Microsoft style drive designation.
A: & B: = First and second floppy
C: = First hard drive
D: through Z: = Other hard drives or removable drives (CD/DVD/USB) or mounted shared drives from another server.
DOS/Windows don't distinguish between uppercase and lowercase so A: and a: are the same as are M: and m:. Linux/UNIX does distinguish.
Given how high the letter is (m
I'd guess it was a shared filesystem mounted with Samba to the Linux host.