I had a simillar issue. The dmesg had SATA 1.5GB messages and decided to shutdown the SATA port. It seems connector doesn't pinch enough to keep a proper signal transport. The NAS was a secure place and hasn't been physcaly touched for a long time.
First,I removed and plug back the impacted hardrive, nothing changed even worst 2 more went down. I did check all voltages and everything was fine.
I solved the issue by using contact cleaner fluid (stays a little bit greasy but transport signals), scrap with a special gum the harddrive connector.Everything went back to normal right after.
My final fix was to use screws to fix the hardrives in place