This sounds like a hard drive serial number problem--your drive may not have a serial number. I've cut and pasted the following from the readme--it describes a slightly different problem, but the solution is the same. Let me know if it helps:
  On some systems with multiple hard drives, all audio files will be properly scanned into the Library
   but those on a particular drive may not be available for playback. This occurs when the Volume Serial
   Number of 2 hard drives on a single system are identical (the Volume Serial Number--also known as a 
   Disk Identifier--is assigned by Windows whenever a drive is formatted).
   Volume Serial Numbers associated with each drive on a system are usually unique within the system, 
   but occasionally, different hard disks within a system will possess the same Volume Serial Number. 
   Since MediaMonkey relies on volume serial numbers to track the location of files, this situation can 
   result in files being unavailable. 
   To determine if this is what's happening, you need to verify the Volume Serial Numbers of your drives.
   From the C: (or D:, etc.) prompt, type dir /p, and on the second line, you'll see 'Volume Serial Number 
   is xxxx-xxxx'. If this is the same for 2 drives, then the problem is most likely due to identical 
   serial numbers for the different drives. 
   The Volume Serial Number of a drive can be changed to solve this problem, using some utilities. One that
   has had good feedback is the Volume ID utility available at:
   
http://www.sysinternals.com/ntw2k/sourc ... l#VolumeId .
This sounds like a hard drive serial number problem--your drive may not have a serial number. I've cut and pasted the following from the readme--it describes a slightly different problem, but the solution is the same. Let me know if it helps:
  On some systems with multiple hard drives, all audio files will be properly scanned into the Library
   but those on a particular drive may not be available for playback. This occurs when the Volume Serial
   Number of 2 hard drives on a single system are identical (the Volume Serial Number--also known as a 
   Disk Identifier--is assigned by Windows whenever a drive is formatted).
   Volume Serial Numbers associated with each drive on a system are usually unique within the system, 
   but occasionally, different hard disks within a system will possess the same Volume Serial Number. 
   Since MediaMonkey relies on volume serial numbers to track the location of files, this situation can 
   result in files being unavailable. 
   To determine if this is what's happening, you need to verify the Volume Serial Numbers of your drives.
   From the C: (or D:, etc.) prompt, type dir /p, and on the second line, you'll see 'Volume Serial Number 
   is xxxx-xxxx'. If this is the same for 2 drives, then the problem is most likely due to identical 
   serial numbers for the different drives. 
   The Volume Serial Number of a drive can be changed to solve this problem, using some utilities. One that
   has had good feedback is the Volume ID utility available at:
   http://www.sysinternals.com/ntw2k/source/misc.shtml#VolumeId .