I have a Pegasus2 TB2 R4 RAID 5 running, and recently changed one drive, the r4 continued to work flawless for some weeks, but now about 10minutes after booting the mac (trashcan 2013, mojave) copying starts to slow down, and finally accessing files is barely possible in the finder. all other hds disconneted, changed tb2 cable, no smart errors. rebooting machine and re-powering up r4 helps all the time.
blackmagic speed test goes down from 600read to 40mb/s read after a few tests.
heres my subsystem info: https://we.tl/t-tgGRMYeoUN