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2024-04-08 12:05:55Z (last updated 2024-05-07 14:48:21Z)
I have an external drive. It's a 4 terabyte drive, and I'm about to use it for backups.
I also have an external SSD. It's also a 4 terabyte drive, and I have filled 500 gigabytes of it with random stuff that I can afford to lose (which I have excluded from backups). I also took 100 gigabytes of it and created another partition for file transfers, so it'll be excluded from backups. That means the external SSD uses 3.4 terabytes at maximum.
I also have a MacBook, and the internal storage space for it is 500 gigabytes. I took 100 gigabytes and dedicated it to an Asahi Linux install, which I won't backup either, so that's about 400 gigabytes maximum on my MacBook.
So in total, that's about 3.8 terabytes. And I'm about to create a backup onto a 4 terabyte.
Now, here's a snippet from the Apple help page for backing up your Mac with Time Machine:
Use a storage device with at least twice the storage capacity of your Mac.
Apple help page for "Backing up your Mac with Time Machine"
"twice the storage capacity of your Mac" would also include my external SSD, because I include that in backups (after learning the hard way). Given my "[maximum] storage capacity" is 3.8 terabytes, my backup drive would have to be at least 7.6 terabytes. However, 4 terabytes is barely bigger than 3.8 terabytes, and there is a long way left before 7.6 terabytes is reached.
However, I can't get a hard drive that's 8 terabytes (close to but more than
7.6 terabytes). Mainly because I couldn't find any.
So how do you make backups onto a drive that's barely big enough?
Recently (very much not), I had to wipe my backup drive again, because Time Machine decided to break again (Time Machine as usual).
I wipe the drive, set it up for backups, and start the backup. All good, right?
Wrong. It doesn't work. Time Machine has run out of space to create backups.
So I started from square 0, wiping the drive again and attempting another full backup. Nope, same issue.
I thought 3.8 terabytes would fit into 4 terabytes. I guess not. Also, it's not 3.8 terabytes, it's more like 3.5 terabytes of actual data. But that should fit into 4 terabytes as well, right?
It should, but for some reason, it doesn't.
So of course, the only obvious next step was to make it somehow work.
Time Machine backups can exclude entire volumes, or specific folders.
I already use that feature to exclude stuff that I can easily afford to lose.
However, what if I (ab)used that to create a smaller backup?
So here's what I did:
1. Exclude my external SSD
2. Create the first backup
3. Exclude a large chunk of my external SSD, but also include a small/medium chunk of it
4. Create another backup
5. Include my entire external SSD (except for the losable stuff)
6. Create the last backup in the process
So the size of the data backed up grows and grows until it is complete. Gradually increasing the size of stuff that is backed up.
Compared to "wipe the drive and create a backup from scratch", that was basically "backup literally everything at once lol", and for some reason that doesn't work well.
I'm not sure why this works, or why Time Machine has troubles creating an initial backup onto a drive with very little space left. Maybe it needs twice the space temporarily for the first few backups? I'm not sure.
The actual used space I've used on my MacBook and external SSD combined is 3.5 terabytes, which is 0.3 terabytes less than 3.8 terabytes.
My 4 terabytes backup drive would be about 1.142857143 bigger than 3.5 terabytes. That number isn't 2 and is much less than 2, so that means the backup drive is not twice as big my data.
(4 terabytes) / (3.5 terabytes) ≈ 1.142857143