First things first. Major props and an internet high five if you picked up on the nod to Huey Lewis/Back to the Future in the article title. By now, seemingly mandated BTTF references while discussing Time Machine have become clichéd. However, I still can’t resist the urge to pay homage to one of my all-time favorite films when the chance presents itself. After all, my own Time Machine drive is named ‘Flux Capacitor.’
Though one of Time Machine’s greatest assets may be its ability to handle the behind-the-scenes work that goes into automating backups, its interface is often scorned for being too simplistic. By default, Time Machine is configured to use the entirety of the drive it’s paired up with. If you’re using an external drive for the sole purpose of backing up, then this isn’t typically a problem. However, what if you want to multi-purpose the drive for other types of files? Assuming the content of your initial backup does not fill your external drive, then you are afforded this option from the get go. As long as you don’t modify the backups.db folder on the disk, you’re free to drag and drop files just as you would with an ordinary hard drive. However, as you continue to backup with Time Machine you will eventually be prompted to delete these files to make room for additional backups. If you’d like to limit the amount of disk space at Time Machine’s disposal, you can run a simple Terminal command to do so. Reader Melle S. from The Netherlands contributed this great tip. Melle writes:
By default TM will use all the space on the backup drive, when necessary. So, what if you want to set a limit to that? It is fair to expect a user to use an external disk for other purposes as well. Time Machine Preferences does not offer you that option. Well, we can do that. With a terminal command. The command is the following:
Suppose you want to limit the backup to approx. 200GB, which is 204800MB:
defaults write /Library/Preferences/com.apple.TimeMachine MaxSize -integer 204800
To remove the limitation, do this:
defaults write /Library/Preferences/com.apple.TimeMachine MaxSize -integer 0
By running this command, you can save only the data you need to–leaving the remainder of your drive for other files. Though this tip may not be right for all users, it is especially convenient if the collective size of your data is well under the size of your backup drive. Let’s say you are using a MacBook with a 250GB hard drive, and out of that space you’ve only used 150GB. If your Time Machine drive is 1TB (1,000GB), you’ve got more than enough room to store a series of Time Machine backups as well as other data. As Time Machine does not backup the entirety of your internal drive when performing subsequent backups, a safe cap in this scenario would be 500GB or -integer 512000 in Terminal. This would give you half of your drive’s original capacity to catalog chronologic Time Machine backups, and half to store extraneous files. While you could just partition your hard drive, this method seems a bit cleaner, and can be reversed at anytime. To clear a disk partition, one must either delete it, or in some cases erase their drive entirely.
Although this command works as described, my mantra–another cliché–is: better safe than sorry. Therefore, if you’re interested in trying this trick I’d recommend that you do so both at your own risk, and on a secondary backup drive. Though Time Machine drives can certainly accommodate files outside of the backups.db folder, there’s something to be said for having a dedicated Time Machine drive for the peace of mind it brings. Personally, I’ve used this technique to multi-purpose one of my portable drives I frequently cart around with my MacBook Pro. I can backup on the go, but I also have access to all sorts of media files. In my opinion, this tip really enables the best of both worlds. Thanks again to Melle for this awesome tip!