Time Machine is a really effective tool for backing up everything on your drive. And when I say everything, I mean it — Time Machine indiscriminately backs up a lot of junk on your drive. And because of the techniques it uses and the way some files are used by your apps, you might find that your 500GB external backup drive is getting full after just a few months of use.

It’s really easy to identify folders for exclusion from Time Machine, which will save time and space during backups. It’s worth noting, though, that if your machine’s drive does fail, you won’t be able to execute a full restore without having some of these locations backed up. Then again, if you’re anything like me, you’re far more likely to just do a full reinstall and just restore your lost user data. (Doing a full restore from backup just doesn’t have many advantages compared to a full system reinstall, anyway.)

These are just a few suggestions — you should NEVER exclude anything from backup if you’re unsure you have another copy, aren’t able to get it again, or are unaware of the possible ramifications. Let me just say that again: if you aren’t completely sure about an exclusion, trust Time Machine to do its job.

Of course, you’ve got any good folders you think should be added to the list, feel free to drop ‘em in comments.

The duh stuff.

  • /Applications – This will likely save you more space than any other single folder, especially if you have lots of apps. Just make sure you know what programs you’d want to get back if your machine were to die.

Caches and downloads
Big directories of files that should probably be excluded automatically, but aren’t. Unless you’re a crazy developer debugging code, you’ll probably never need a backup of your caches.

  • /Users/[user]/Library/Caches and /Library/Caches – Between the two you can knock off a few hundred megs of constantly changing, essentially useless data (for example: all the page caches from Firefox).
  • /Users/[user]/Downloads – Where all your internet downloads wind up. Frequently changes, and if you’re anything like me, it’s filled with gigs of garbage.
  • /Users/[user]/.Trash and /.Trashes – Some people might see value in backing up their trashed files. Not I.

More after the break.

Audio and media
Some more obvious picks. You probably don’t need to back this stuff up.

  • /Library/Audio – You can save from the hundreds of megs to gigs here. Keep an eye out for the GarageBand samples, they weigh a ton.
  • /Users/[user]/Music/iTunes/iTunes Music/Podcasts – I don’t really re-listen to podcasts, but even if I did, they can be re-downloaded easily. So the 3GB+ I save here is a no brainer.

Time Machine un-friendly apps
The data produced by some of these apps can easily be the worst offenders when it comes to backups. Until they get their act together, they should likely be excluded categorically.

  • Parallels (/Users/[user]/Library/Parallels) and VMware (/Users/[user]/Documents/Virtual Machines)- You’ll definitely want to keep a copy of your virtual machines, but if left included in Time Machine backups, your system will continuously save the virtual disks each time you use Parallels or VMware. (VMware has become a little more TM-aware though, which is good.) That means potentially hundreds of wasted GB — not to mention tons of lost time during backup.
  • Entourage (/Users/[user]/Documents/Microsoft User Data/Office 2008 Identities) – Same deal here, Entourage uses one giant, constantly changing file to store all your mail. It sucks not to have your email backed up though, so be sure to use something other than Time Machine for keeping a copy handy in case of emergency.
  • /Users/[user]/Library/Mail Downloads – Whenever you open an attachment in Mail, it stores a copy of the file in your Mail.app attachments folder. If you’re a heavy Mail user, excluding this will save you some real space.

Beware! You likely won’t screw up anything by excluding this stuff, but if you run into trouble these are some pretty important folders. Note: you’ll need to hit “show invisible items” to exclude some of these.

  • /usr, /sbin, /private, /bin – Collectively contain about a billion essential system files that can ultimately just be reinstalled in case of disaster.
  • /system – Root system folder. Contains lots of stuff essential to OS X — gigs of it, in fact. Good place to save some space.