If you haven't ordered the HDD yet, I suggest going BIGGER than 4TB... mostly because bigger storage is dirt cheap and more TM space just means a richer history of backups before the oldest ones need to start getting deleted. With 2TB on your Mac, I'd multiply by at least 3 = 6TB but might even go towards about 10GB just for overkill.
Through one lens, TM is about backups. But through the other, it's about going back in time. More space, gives you the ability to go back further than less space. Where would this matter? A corrupt file will back up too. If you have the ability to go back in time, you could step backwards to the last version that was not corrupt and recover it.
Example: you have a big document- let's make it a book for a very tangible example. Somewhere doing the process, you accidentally delete finished chapters 4 though 8 but don't notice because you are working on latter chapters. Weeks or months later, you have some reason to refer back to those chapters and discover them missing at that point.
In a TM backup drive, each update to the book is going to be "remembered" in a backup. So you might have- say- 50 backups since the accidental deletion. #51 has the latest version of the missing chapters. So you can take the latest version of the book as one file, recover #51 to regain access to finished Chapters #4-#8 and then drop those chapters into the former for a complete book again. If your TM backup drive is:
- Small: maybe it runs out of space before it can back up 50 iterations. If so, #51 and later are auto-deleted to make room for even newer version backups. Chapters #4-#8 are toast and you'll need to recall and rewrite them again from YOUR own (biological) memory.
- Big (enough): there may be 100 or 200 versions-in-progress backed up, so you have the easy ability to recover the missing chapters from iteration #51.
As the name directly implies, going back in time is a key feature here. In a hypothetical real Time Machine you build to go back and correct some mistake or change some past event, if your creation can only go back a little ways in time, you may reach a point in the present where going as far back as it is able is not far enough back. For example, if the event you want to change occurred 40 days ago but your machine is limited to 30 days, you can't use it to correct whatever went wrong. Same here (only this works in backed up VERSIONS of files). In this rough analogy, time is still fluid up to 30 days back in time but then becomes rigid beyond 30 days. In this TM app, after you run out of space for more backups, the oldest ones are deleted to make new room (thus your ability to travel back to their time- their
versions- is being destroyed).
Another reason to go bigger: if anyone lives with you and has a Mac too, add your 2TB to whatever space they have, multiply by 3+ and consider backing
BOTH up to that TM drive. For example, if the other person had 2TB too, 4TB times at least 3 = at least 12TB target size for the TM backup
One MORE smart consideration: make that
TWO bigger HDDs to both be active TM backup drives and store one of them
offsite to protect against fire/flood/theft. Regularly rotate onsite one with offsite one so that the offsite one is pretty up to date. I do this every 30 days and store it in a cheap bank safe deposit box. But any secure offsite location could suffice.
50 perfect backups all stored in one place could all be lost in a single fire/flood/theft scenario. Get
ONE recent backup
offsite and your ability to recover almost everything grows dramatically.
