Same issue after upgraded to RouterOS 6.28 mipsbe.
Reboot can fix this issue on RB951G-2HnD, but make no effect on my RB751G-2HnD.
6 sector writes per second.
Having the same trouble after updating one of my RB951G-2HnD to 6.28: around 2 sector writes per second. Reboot seems to help to make it stop.
After messing around, I discovered that once WinBox is closed, the counters are not growing anymore. In other words, you can disconnect WinBox, log in a few minutes later, and discover the same sector write count. It starts to grow again at the same pace when I open IP → Firewall → Filter/Mangle view in WinBox. Closing IP → Firewall view still keeps the counters growing. Closing the WinBox session, and connecting again (IP → Firewall view closed!) stops counters from growing. I would guess some background thingie that reads firewall stats is writing on disk.