After making room for files to be written, the bug disappeared. Therefore, I can conclude that the bug was most likely caused by insufficient disk space. When the hard disk is nearly full, but Ubuntu One tries to sync new files anyways, it runs out of space which apparently was what caused the issue. So here's my suggestion: Since I'm somewhat experienced with programming, I think this could easily be avoided by using a try/catch statement when syncing data. If it fails to read/write data, have a notification shown to the user that Ubuntu One failed to write data to the disk due to [reason, such as lack of file space]. This could be done similarly to how Empathy alerts you of the topic when you join an IRC channel, or when you change the volume and get notified of it. Unfortunately, I'm not quite sure how to code this, so I'll leave it up to someone else.