When trying to access the MobileMe iDisk in the Finder, you may run into a problem in which the disk will not mount and will give an error stating "Your iDisk is unavailable." This problem may crop up after applying software updates or reinstalling the system, but it may also happen after syncing with new mobile devices or computers, or even seem to happen randomly.
If this happens, there are a couple of things you can try to fix the problem:
Sometimes a basic restart of the system will be enough to fix the problem. Usually just logging out and logging back in would work, but restarting can be another way to ensure system processes and services are refreshed.
Log out of MobileMe
Go to the MobileMe system preferences and log out. Then log back in and try accessing your iDisk again.
Use a full MobileMe e-mail address as the log-in
The mobileme username should work when accessing the MobileMe service; however, some people have had success by changing their username to be the full "firstname.lastname@example.org" e-mail address.
Remove Keychain entries for MobileMe and iDisk
Go to the "Keychain Access" utility and use it to locate and remove keychain entries associated with MobileMe or iDisk. After this is done, go to the MobileMe system preferences and log back in to MobileMe and try your iDisk again.
Change your MobileMe password
Go to the MobileMe Web site and change your password for the service. Then try logging in with the new password, and you can then optionally change the password back to the old one.
Keep in mind this problem may be due to several of these factors, so you might try doing them all by first logging out of MobileMe and then removing the password from your keychains. Then go to the MobileMe Web site and change your password. After this is done, restart the computer and then supply the new credentials to the MobileMe system preferences and try accessing your iDisk again. If this works, you can then try changing your password back.