When MobileMe goes away at the end of the month, iDisk will go with it. Before that happens, you should copy any files from your iDisk to some other service.
iDisk is a virtual disk provided by Apple. When connected, it shows up on your desktop as if it were a hard drive, but the files you transfer to and from it are transmitted over the Internet to Apple. The advantage of this approach is that these files do not take up space on your drive and are available from anywhere in the world. The drawback is that it’s slow.
If you’ve never used the iDisk, don’t worry about it as it will be gone soon anyway. Otherwise, a great alternative is DropBox, a free service that provides a type of synchronized disk.
The DropBox software creates a folder on your hard drive that acts like any other folder except its contents are synchronized over the Internet to the DropBox service. If you add the same account to another machine (Mac or PC), the same files will show up there automatically. You can also access the files online through their web site and use it to share large files with others.
The drawback is that it takes up space on your hard drive so it isn’t truly virtual. But it is pretty fast, and it isn’t going away.