Two years ago, I got a Drobo for my birthday. It is still serving me well.
Initially I populated it with 2 400GB drives.
That gave me 371GB of protected storage.
Then I added a pair of 500GB drives.
Bingo! 1.2TB of protected storage.
Looking towards Santa's visit this year, I bought a 1TB drive on Black Friday. To install it, I just popped out one of the 400GB drives. The remaining 3 drives began blinking orange. I slid the 1TB drive into the now open slot. In a few seconds all 4 drives were blinking orange and green. After about 6 hours, they were all green again.
This only took me to 1.3TB of protected storage due to the asymmetry.
After Santa makes his visit, I'll replace the remaining 400 GB drive with a second 1TB drive.
Ah! 1.8TB protected storage.
That's nudging right up against the 2TB limit of USB 2.0. Although the Drobo will present 2 drives to the host if it goes over 2TB.
But wait, if I replace one of the 500GB drives with a 750GB drive I get 2TB protected storage.
PS. Here's the Drobo Capacity Calculator.