Oh such a common problem these days with the Sy77. I'd say 95% of the postings about the sy77 in the last year has been about the disk drive and 100% of these problems have been the belt in the drive is worn out and needs replacing. $300, no way! I spent $15 to fix mine. Check the link below. It is my response to Snoids describing to him what I did. http://www.synthworld.com/ubbs/Forum26/HTML/000573.html
Also, when you get it fixed you will want to get all your sy77 files converted to sysex files so that you won't need the drive, anymore. Currently, I only know of a way to convert the sounds in the s77 format to sysex which can then be midi'd to your keyboard using your pc. I haven't found a way to convert both sounds AND song data. If you are only concerned about using sounds and not losing any song data then check this site http://www.neuroinformatik.ruhr-uni-bochum.de/ini/PEOPLE/heja/sy-computing.html
It has a file called "Syconv" that will convert Sy77 files to a sysex file with the sounds in it. No song data will be converted though. The sysex file can then be midi'd to the sy77.
Hope all this helps.