Messed up symbols

Hi, I posted this in 2500 JJ forum but didn't get a response, not certain if it is caused by JJ, I have a 2500jj and 90% of the time things are fine but every so often, if I am browsing files on the hard drive or (in the worst cases) saving files, all the text and folders are messed up, instead of letters it's just a load of meaningless symbols, so I cant browse folders, and usually the right amount of folders aren't there, just a moment ago there were only 2 folders in Root which were just symbols and I couldn't get into either of them. Has anyone else experienced this, or is my hard drive dodgy? I've had the same problems on and off for years so I don't usually blame the hard drive. Any ideas?

Thanks!

Did you make sure to format your hard drive from within the mpc?
I've never had this problem (I've used all JJOS versions over the years and am on 2XL).