Pardon me if this has already been posted-I couldn't see it anywhere.
Having just nearly found out the hard way, when fitting a N90s body that has been used with film to a DCS 400 series back, apart from the obvious battery compartment/film back removal, there is also a small screw that needs removing just above the shutter as per photo below.
I don't think I damaged the CCD, but the photo's came out blurry as the CCD couldn't sit flush with the camera body! Presumably it is there for film alignment.
_________________DCS serial no. survey
EOS-1D Mk I & II, EOS*DCS-3 & 5, DCS, DCS-200ci, 420, 460, D1, D1H & X, D2H, E2, F, F2, F3, F4, RD-175, EF 50mm f/1.0, Audi S2, Porsche 911, GSX-R1000 K9