Probably impossible to get that password as it could even be unit specific. You might have luck if you use an extension cable for the hdd so that it is external from the GPS unit, then swap the SATA cable (don't touch power!) on-the-fly and plug it in an eSATA port on your laptop. Using a master password will unlock the hdd, but you won't be able to retrieve or access the contents on the hdd as using a master password will format the hdd.
Another way is to use an unlocked PCB (just swap the whole thing), since the password and locking are only stored on the PCB and the actual data on the platters is not encrypted by in any way.