RT-11 doesn't recognize my 3.5" floppy

I have installed an RQDX3 and the M9058 distribution board in my 11/23+. Since I don't have a 5.25" drive yet, I hooked up a 3.5" HD (1.44 MB) drive from an old PC. After a struggle (which I documented on VCFED's DEC forum), I managed to get all the jumpers and cables set correctly, and now my XXDP diagnostics (ZRQA?? ... ZRQF??) recognize the drive as an RX33 (DU0:, logical drive 0 since no hard disks are attached). It passes all the tests, and I can INIT, DIR, and copy files to it using the limited OS with the XXDP suite. The LED on the RQDX3 blinks once when the drive is accessed. So far so good. But, when I boot the system (with RT-11SJ V5.01), it can't see the drive at all. Attempts to access it result in the command hanging indefinitely, the drive does not select, and the RQDX3 lamp flashes rapidly. SHOW DEV:DU does say that the handler is installed for the correct 172150, 154 location. However, SHOW DEV:DUn where n=[0..3] displays two blank lines then back to the dot prompt. Is my version of RT-11 just too old to recognize an RX-33? If so, what do I need to fix this? Presumably a later DU.SYS? Thanks for any help. Most of my experience is with PDP-8's so this is slow going... -Charles