Ok. 2 things are common with Hitachi drives with the unreadable disk error.
#1 Fix, Magnet came loose
Get the drive apart and let's look at the top of the case:
The magnet that comes loose, comes from here:
If the magnet is your problem, it will be here:
If the magnet is NOT there, this is not your problem, proceed to Fix #2
If the magnet is your problem, simply add a small amount of super glue and re-glue it to the spindle. Make absolute sure that it is flat, flush, and is completely back into the spindle.
Sometimes there are holes in the magnet that little nubs inside of the spindle go into. If by chance the nubs have broken into the holes of the magnet do this: Take some sand paper and sand the back of the magnet flush where the nubs might be sticking out. Next, take a micro screwdriver and scrape the nubs out of the spindle. They break easy and it is easy enough using a small screwdriver to scrape them flush. Proceed with gluing.
#2 Fix, re-lubing the motor
NOTE: THIS FIX ONLY WORKS ON HITACHI DRIVES.
Locate the motor, shown in this picture. I have also pointed with screwdrivers to the screws that will need to be removed.
NOTE: I did not have a Hitachi drive on hand, this is a Samsung model, but the concept is the same and they look very much alike.
Remove the 3 screws, be careful not to lose the springs under them.
Next, CAREFULLY remove the ribbon tape from the circuit board:
Now the motor should come out easily:
Now it's time to remove the spindle from the wire coil inside:
Grab onto the spindle (the black part) and pull up and away from the silver deck it is attached to. It will come off fairly easily. On the bottom of the spindle there will be a short metal bar. Thoroughly clean the bar and inside, also inside the hole the bar fits into. Use basic 3 in 1 oil reasonably on both the bar and in the hole.
DO NOT use WD-40 or like items, they will gum up and seize the motor down the road. Reattach the spindle by simply pushing it back in the hole. Spin the spindle a few times to assure even distribution of lubricant. Put the case back on and test.
If this does not fix your problem, your motor is beyond repair and at this pont you will have to replace the drive unless you can find another motor, but they are hard to come by and I wish you luck
If you do replace the drive, you WILL have to get a Hitachi model the same as yours and you WILL have to remove the circuit board from the old unit and exchange it for the circuit board in the new unit. This will keep you from having to do a reflash.
Hope this help and Good Luck!
Steve