Hello,
I have used the Technistar K4 ISIO with 4 or 5 TB SATA hard discs from time to time without any problems. They simply had to pass the ISIO DVR suitability test, which can be found in the settings menu of the K4 (Setup -> Manage films-DVR, music, pictures -> Recording media settings -> Check DVR suitability). In my opinion, SSDs should also work, but they were too expensive for me at the time.
I formatted the discs to NTFS, as FAT can only hold 2 TB. I've since gone back to 2TB discs because the hard drives are shock-sensitive and I was afraid of losing too much data at once in the event of problems, but I never actually had any.
I hope this helps, good luck!

I'm using it with a USB hard drive (not an SSD) with a 4TB external power supply without any problems.