Does the Mount menu option in Disk Utility succeed in mounting it?
With an HFS+ formatted drive the solution would probably be achieved by repairing the drive's directory with DiskWarrior.
At bootup/close-down OS X does a drive-maintenance pass on all drives, which probably effects a temporary repair and gets the drive mounted.
Whilst using FAT32 for simple file transfer is fine (where a backup/re-initialise can be done at the first sign of a problem), for long-term project use its advisable to use HFS+ drives on a Mac - for exactly the problem you have discovered (or worse, with some sequencer software).
Use MacDrive or MacOpener to read the Mac-formatted drive on a Windows PC.