One place where backup programs really fail is in backing up large files. If, say, your run your VMWare machine, thus updating its disk; you add metadata to a high quality ripped DVD video; or you use Microsoft Entourage, so every time you get a new email, your giant Entourage database gets changed.

Some backup programs can handle block-level updates, so that only the chunks of the file that have changed get backed up, but most don’t.

Well, this little trick on MacOS X Hints is genius! Apple’s Disk Utility in 10.5 lets you create “sparse bundle” disk images. These are just like normal disk images, except that they break up the image itself into numerous smallish files, each one is about 8 MB.

So you can take your big ol’ file, put it into a sparse bundle, and poof! You can now handle backups and it’ll only change a handful of 8 MB files.

While the hint I’ve linked to is pretty complicated, you can just have the image and put large files on it and not worry about the rest of it. You can even just make an alias or symbolic link (type “man ls” in the terminal for a how-to) from one folder to another (yes, this does work on the “Microsoft User Data folder), rather than have a complicated script.

Note that, a few programs that generate big files offer similar features. VMWare, for example, lets you split up your VM’s hard disk image into 2 GB chunks. Not quite as minute as these sparse bundle backups, but still quite good (and probably faster, which is a must-have for a VM). Also, if you’re using FileVault, you already have a sparse bundle containing all your files on your hard drive. Nifty!