Time Machine backup question 1
February 4, 2019 11:19 AM   Subscribe

Can I back up my Macbook to an external drive that has other files on it, without affecting or erasing those files? I have a nice solid state external drive with plenty of room on it. Can I use it for a new Time Machine backup of about 300gb without hurting the files that are already saved to it? How to do it if possible, just put the backup in a new folder on the external drive?
posted by glasseyes to Computers & Internet (6 answers total) 1 user marked this as a favorite
 
Yep, you can do this, Time Machine will just create a "Backups.backupdb" folder on the drive and backup to whatever the capacity of the drive is. When it's out of room, it will delete old backups to create new ones.

HOWEVER if you want encrypted backups Time Machine will erase the drive, so... don't do that. (At least that's how it used to work.)
posted by Automocar at 11:47 AM on February 4


From my experience, Time Machine is a laughable excuse for a backup system. Unfortunately, there doesn't seem to be any real alternatives in OSX.x that are "free" other than just manually backing up important files yourself. Forcing you to allocate an entire drive to backup is clunky at best and can be a management nightmare at worst.

My personal solution was to just use 3rd party imaging software* to stamp out a clone of the aforementioned system drive on a semi-regular basis and back that up to some sort of removable media. YMMV.

It can't be overstated that you should always backup important data.

Good luck.

*I used to use Carbon Copy Cloner. Glad to see it's still around, and even better to see that it seems to have some increased usability.
posted by Sphinx at 12:28 PM on February 4


I just did this and it worked out fine. I wanted to wipe my secondary drive on my laptop to reformat it to AFPS and did a Time Machine to a spare external and then restored the newly formatted drive. It worked fine.
posted by jadepearl at 1:22 PM on February 4


I'll note that the last time I did this, if the destination disk was remotely close to the size of the source disk, the destination disk would gladly fill up with Time Machine backups to the point that you wouldn't have any space left for any other data. i.e. if your main disk is 1TB and your external disk is 1TB and you have any reasonable rate of change to your source disk, Time Machine will fill up that external disk almost immediately. It won't delete your existing files, but you'll be sneaking crumbs left over from the backup rather than sneaking a backup onto an external drive. On the other hand, if you just want to take a single snapshot and turn off time machine on that disk, that's not a problem.

If you want to keep using Time Machine to back up to that external drive regularly, you can usually losslessly fiddle with a disk in Disk Utility (Applications/Utilities) and shrink the existing partition to a reasonable size and make a new partition for Time Machine's exclusive use. Not always, but it usually works, assuming the disk was a reasonable target for Time Machine to begin with. That way Time Machine will rotate backup sets within the new disk size, leaving a guaranteed volume size for documents/whatever else was on there originally.

(Which is to say: you definitely do not need to allocate an entire drive to Time Machine, but most people are probably best suited to allocating an entire partition on a drive.)
posted by Kyol at 3:17 PM on February 4


I have a nice solid state external drive with plenty of room on it. Can I use it for a new Time Machine backup of about 300gb without hurting the files that are already saved to it?
Among other things, this will depend on the current type of filesystem on the drive. I think the drive must be an HFS+ filesystem [aka "MacOS Extended (Journaled)"]. If your external drive is formatted as something like FAT32 or NTFS I don't think you will be able to use it as a Time Machine disk without reformatting.
posted by Nerd of the North at 3:58 PM on February 4


Thank you everybody. So I see it can be done, I'll proceed cautiously and check if Disk Utility gives me some nice options as well. Cheers.
posted by glasseyes at 6:04 AM on February 5


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