What capacity do I need for Time Machine

Rule of thumb is that the Time Machine volume should be twice the size of the data it is backing up.

 

Another rule of thumb is that don't share the same partition with other data. I experienced a number of times that an improperly unmounted Time Machine drive developed filesystem errors beyond repairs (and had to reformat).

Another is that you shouldn't depend Time Machine to archive things for you. Keep another drive for archival purposes (or at least another partition).

 

It depends on how full the 2 Tb drive is, and how frequently you update or change files on it. 4 Tb would be more than enough for most people. My old iMac had a 1 Tb drive and I used a 1.5 Tb drive for Time Machine. Over the course of several years, I only used about 800 Gb of the TM drive.

 

There's a comprehensive guide on Time Machine by James Pond and it's the best resource on Time Machine out there. He states "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)."

Would highly recommend checking it out: http://pondini.org/TM/1.html

 

Technically, you only need as much space as you are trying to back up or as much as your main drive can hold. Your system has caches worth gigabytes and other things that are never backed up. Thus a 2 TB drive would be enough to make a backup for a 2 TB system. The only reason why you should use more is to make use of snapshots. Depending on your disk usage, filling up the remaining space can take weeks or months or even just an hour if you make very big changes.

I only use a ratio of 1:1.2 of my drive’s size, as my main disk is never filled anyway and I don’t really need the snapshots, aside from a buffer for system upgrades. I still have two months worth of snapshots.