Let's say I have two 2TB HDDs and I want to start my first ZFS zpool. Is it possible to create a RAIDZ with just those two discs, giving me 2TB of usable storage (if I understand it right) and then later add another 2TB HDD bringing the total to 4TB of usable storage. Am I correct or does there need to be three HDDs to start with?

The reason I ask is I already have one 2TB drive I'm using that's full of files. I want to transition to a zpool but I'd rather only buy two more 2TB drives if I can. From what I understand, RAIDZ behaves similarly to RAID5 (with some major differences, I know, but in terms of capacity). However, RAID5 requires 3+ drives. I was wondering if RAIDZ has the same requirement.

If I have to, I can buy the three drives and just start there, later adding the fourth, but if I could start with two and move to three that would save me $80.