Easy to FIX

Right click on my computer
Choose MANAGE
Then find DISK MANAGEMENT
You should see your ext hard drive under the list.
Right click on it, and choose CHANGE DRIVE LETTER
Choose anything, but I recommand something higher than M:

I've seen this problem with alot of STORAGE DEVICE

It happen more often with Laptop because they usually have Card reader.
So it makes couple drives like e:, f:, g:.
Storage device tries to install themself with the drive letter next to CDROM
c: - HD1
d: - HD2 or CDROM
e: - Mass storage device (maybe for card readers)
So your external HD tries to have the E: drive (probably a bug)

See ya