Yeah, connecting the disks directly is definitely worth a try. The backplane does not have an expander. The chassis is a Chenbro RM21706. I found a picture of the 6-disk backplane on the web:
In my system each of the connectors on the right is connected to the mainboard with an SFF8087-SATA cable (2x4 SATA connectors total, 2 of them are not connected to the mainboard)