I use Apple's Backup.app to back up files to my iDisk, and this has been working successfully for years. Recently, though, I started getting backup failures. After a lot of head scratching, it seems that Backup was failing because of a series of files that were user read-only, and had no permissions at all for group and other.
Quite why this should matter at all, I've no idea -- especially because the files get bundled into a DMG, and it's the DMG that gets uploaded to iDisk, yet it was the upload bit that was failing. Maybe keen commenters can get to the bottom of it!. Anyway, I've not done any further proofing, but suffice it to say that running the following Terminal command on the directory of files to back up solved the problem:
$ chmod -R +r .Alternatively, you could use the Finder's Get Info window. (This is all on OS X 10.6.1, by the way.)
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091017003501125