I just installed 4.02 over 3.x and had the same message (with M:\) on first launch. Complete maintenance before upgrade.
.db file is on a ramdisk which is not mounted and music files are on the regular datadisk, not mapped nor mounted.
M:\ obviously looks like an absolute path. Rings no bell either: I never used that letter in years.
FWIW, the upgrade procedure took like 20 secs, created an "mm.db.Pre4" file which has the exact same size as the original MM.db (300mB). Looks like a copy to me, hence unusable to 4.x.
Funny my only complaint with MM over the years was with the unique drive id: I might have change hard disk 20 times since 2.5 ... I tried to mount a dedicated volume once, which MM seemed to be fine with, but I hate dedicating disks and waste resources.
Putting .db on a ramdisk saves disk access and some fragmentation, which MM does not like at all. Even without a ramdisk, I'd definitely store the db file outside of the WinDisk : this is valuable data that has to be backed up with the rest and has to be safe from a system restore.
I was hoping 4.x would be the time to get rid of this long time nail in the foot of one category of MM users : the one I belong to
No luck this time. I'll look around before reverting, see if I can bring some clues, but the program looks unusable as such. One potential clue might be that I chose to install in a new folder instead of overwriting MM3 with all its incompatible scripts. Not sure this will help.
Happy end of the ... year