Mac – Time Machine backup of an external drive (to another external drive)

You probably want to check out this website for the most information, but here is the answer to your question, quoted from the website above:

1. How big a drive do I need for Time Machine?

A general "rule of thumb" is, to keep a reasonable "depth" of backups, Time Machine needs 2 to 4 times as much space as the data it's backing-up (not necessarily the entire size of your internal HD). Be sure to add the size of the data on any other drives/partitions you want to back up.

But this varies greatly, depending on how you use your Mac. If you frequently add/update lots of large files, then even 5 times may not be enough. If you're a light user, you might be able to get 1.5 times to work, but that's subject to problems any time a large backup is needed.

And, of course, the larger the drive, the more old backups Time Machine can keep for you. A drive that's too small may only have room for a few weeks (or even days) of backups.

Unfortunately, it's rather hard to predict, and most of us have a tendency to add more and more data to our systems over time, so if in doubt, get a bigger one than you think you need now.

Also, there are some OSX features and 3rd-party applications that take up large amounts of backup space, for various reasons. See question 9 for details.

This is a trade-off between space and how long Time Machine can keep its backups, since it will, by design, eventually use all the space available. But it won't just quit backing-up when it runs out: it starts deleting the oldest backups so it can keep making new ones. Thus, the more space it has, the longer it can keep your backups.

If your backup disk is on the small side, and Time Machine needs to do a very large backup, either because you've added or changed a lot or done something like an OSX update since the previous backup, you may get one of the messages in Troubleshooting item #C4 (which one depends on exactly what happened, and which version of OSX you're on.)

My Answer

Then it'll only take a snapshot that's about 120GB in size, and while that same snapshot will grow over time as I take more recent snapshots of my system, it will never go beyond 500GB because that's the maximum size of my internal HDD.

No, not quite. The 1 TB hard drive will be filled up because Time Machine keeps your backups and deletes them once your hard drive is filled. There is more than one backup stored on that hard drive. As stuffe pointed out as well, Time Machine allows you to restore to a previous backup, since Time Machine may keep 7 or 8 backups on that 1 TB hard drive.

I'm asking this because I want to know if I should use a separate external HDD for storing my actual files.

Yes, you should. You should be dedicating a drive to Time Machine. Here is the quoted answer from 3 in the website I linked above:

3. Can I use my Time Machine disk for other stuff?

Yes. Time Machine will not delete anything you put there. But it's not a good idea to put anything else important on the same physical drive, unless you back it up elsewhere. When (not if) that drive fails, you risk losing it.

If you want to do this anyway, it's much, much better to partition an external drive into 2 (or more) parts, also called volumes. Assign one to Time Machine, for its exclusive use for backups; use the other partition(s) however you want. To use a new drive, or one you don't mind erasing, see question #5. To add a partition to an existing drive that already has data on it, see question #6.

I have seen this on several occasions and I have done this to clear up the corruption or glitch that prevents browsing local snapshots without an actual full time machine destination mounted:

1. Connect to one of my destinations and force a good backup

   tmutil startbackup --block --rotation
   

2. Disable local snapshots

   sudo tmutil disablelocal
   

3. Enable local snapshots - being sure the cleanup is done

   sudo tmutil enablelocal
   

I do watch the console app (or tail -f /var/log/system.log) during the backup and commands above to make sure that the local filesystem gets cleaned up and give the computer several minutes between the disable and the re-enable since it does have to unmount the network filesystem, delete old copies of files and clean up things before trying again with a local snapshot.

The downside of this is you do lose any copies of files that only existed on the local snapshot intervals and didn't get saved to an actual remote location. It also doesn't really dig into why the store became corrupt so you might have the same thing happen again at a later date. I've also had good luck with reproducible time machine problems reporting it to Apple for assistance. They'll want you to run tmdiagnose and probably not reset things like I mentioned above if you want to get at the root cause of what really happened as opposed to getting it working again.