Options for Adding Storage

[ere109]

My server has 12 drives: 8 spin drives, and 4 SSD. All drive spaces are in use, and most of my drives are over 80%. I'm out of SATA ports, and my mobo is USB2. It's time for more storage. As I see it, I have three options: add an ESATA card and an external enclosure; add a USB3 card and an external enclosure; or systematically pull my five 8tb ZFS2 drives and replace them with 12tb. The former two seem like better options, but I'm completely open to new ideas (like adding a backplane to split current SATA ports). Ideally, I'd add four to six new drives and create a new ZFS2 pool, then split my data between the two pools. But I'm open to ideas and discussion on the benefits and drawbacks of various options - speed, power, ability to control individual drives from my FreeNAS GU etc.I