What is largest Drive Size for a Gen2?

I own 2 Gen2’s and would like to upgrade to a NAS model but it was a huge failure 2 years ago so I’m wondering if the Gen2 can accept a drive size larger than 4GB. I’m running firmware 1.4.2 [1.254.50341] on S# TDB121120070 - but Drobo won’t let me register this one because it says it’s already registered. Currently it’s got 4x 4TB WD drives in it, but the top drive is indicating that it should be replaced with a larger capacity drive.

I’m pretty sure I’ve had 6 TB drives in my Gen 2. I can’t check to confirm that since I’ve switched those over to a Drobo S.