How much HDD space do you have left? Some people have had success after defragmenting their HDD, so try that. Have you changed the locations of your saved or shared folders from the defaults (one person got rid of the 'disk problem' error by reverting their folder locations back to default)?

Check out these threads for more info

Finish Download / Disk Problem error msg
The "disk problem" problem
Disk Problem??
disk problem
Low Disk Space Message