Ste,
It is a common thing right now (11 years old) for the SY77 disk drive to break but in every case I've heard of, it is the belt that turns the drive that is worn out. It eventually gets too slick and just starts slipping. You can replace it for under $10 dollars if you're willing to do the work yourself. Do a search on this board for "WhiteNoise". I have an earlier post with instructions on how to do it.
Also the SYCONV file will only convert the Voice part of the ALL DATA file to sysex. I could not get it to convert the actual sequencer data. So if you have any song on disk, you won't be able to convert them with this file.
Once you get your drive back up, then you should convert all your txx files into sysex just in case your drive has more severe problems one day that you cannot fix.