Vista - Hard drive stereo not working with vista 64 bit

I have a hitachi ax-m 135 which is a stereo containing a hard drive. the unit
was formatted and worked fine with windows xp and could be used as an
external hard drive would be.
Ive now migrated to Vista Premium Home 64 bit, and the stereo is not
recognised. I connect with a usb cable, and vista sees the stereo connecting
but then asks for drivers (non were needed with XP) which i dont have. ive
tried all the ways of doing the install ie automatically, and manually
searching searching for a driver but it wont work. on one occasion the hard
drive was seen but vista stated it needed to be formatted to work, i really
dont want to do this as i will loose 20+ GB of music which on this stereo has
to be very carefully arranged.
Any ideas would be very well received and save me alot of hassle and money