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.
Basically, I would recommend not trying to constrain time machine to only a weekly interval on the offsite drive.
That being said, you can have both drives work totally independently as that is how the software is engineered. If you really only want a weekly snapshot, only connect the weekly drive once a week and force a rotation of the destination:
tmutil startbackup --rotation
Once you have a backup to the weekly drive, eject it and send it away. As long as all dives get reattached every 14 days or so, you won't get a warning dialog that any are out of date.
Again, you can certainly enforce a single weekly backup by the procedure you follow but two things make that practice less needed than you might expect:
- The backup intervals are very low space overhead compared to the size of cheap backup disks.
On my worst Mac, the system averages 5 MB of overhead for system log files so in absence of user files changing, you have 24 daily and 31 monthly snapshots you would avoid storing 275 MB (or 0.0003 of a one TB drive) if you were on a strict one per week schedule.
Since the cleanup of old backups is so fast and the storage impact so low, most clients with offsite rotation end up just swapping disks each week and let the software cull the extra snapshots per the normal schedule, leaving weekly snapshots more than a month old.
Best Answer
It seems that local snapshots are enabled automatically in High Sierra.
The text in the red box translates to "local snapshots, as long as disk space is available".
I didn't find something from Appe on this, but this blogpost comes to the same conclusion.