This may be a long shot, but Hushi *may* be the culprit.
My Falcon had a very similar configuration until I discovered that Hushi was not working 100% reliably with my internal IDE harddisk.

There's an option in Hushi's config tool though that you could try:
- in SCSITOOL.APP go to the "Treiber" (driver) menu
- select "konfigurieren" (configure)
- click "IDE-Parameter"
- disable the "R/W multiple" option for all your IDE partitions

This made my IDE drive working stable on my Falcon, although there's some speed penalty when this option is off.

In the end, I just dumped Hushi in favour of HD Driver, which works just perfectly with all my drives. :-)