Every 72 hours after you start up or restart your computer, a routine maintenance task (or "cron job") runs that can affect iDisk. If you have selected the "Create a local copy of your iDisk" option in .Mac preferences and cannot sync after your computer has been on for three or more days, download and install the Mac OS X 10.3.1 Update or later.
Similarly, if the Backup, Library, or Software folders of your iDisk appear to be empty, download and install the Mac OS X 10.3.1 Update or later.
Note: The task should run at about 3:15 a.m unless the computer is in sleep. You might be able to avoid this issue by making sure the computer is in sleep at this time, however this will prevent other background maintenance tasks from running, which are described in technical document 107388 "
Mac OS X: How to Force Background Maintenance Tasks (Logs and Temporary Items)".