If you recently lost about 10GB of hard drive space on your Mac, it may be because the iDisk just increased it’s size (up to 10 GB). If you would like to get this space back, turn off “iDisk Syncing” under the iDisk tab in the .Mac preference pane.
The local copy of the iDisk apparently carves out 10GB of your HD, just to make sure that it can store what you send to the iDisk.
After turning off the Syncing, you have to delete the “previous idisk” image that is on your desktop (it’s 10GB).
You should then have your 10 GB back (or slightly less, depending on how much info you had in your iDisk before it expanded)