Have the 12tb version which went through two separate drive bay failures before Pegasus software updates fixed overheating problem. Drives replaced without charge as under three year warrantee . But after warrantee ...yikes, expensive. Tech support good and no problems since then. Working on at 2011 iMac with Mtn Lion. Crazy prices though for these boxes! Daisy chaining separate 4tb boxes seems only other option for normal folks sadly
I added 4x4TB Hitachi drives. Worked well in two Raid-1 arrays. Then I migrated all 4 to RAID-5. Promise Utility converted all 4 to a RAID-5 and correctly found 12TB/16TB to be usable.
HOWEVER, OSX Mtn Lion does not recognize this size of RAID and gives back a 4TB limit. Frankly, this looks like an OSX glitch more than a hardware problem--but it is a major impediment to maxing out storage.
Contact with Promise Customer support was desultory and unhappy. Web support just said 4TB drives are not supported. No offer of workaround, nor prospect of firmware revision.