HR21 won't use eSATA (SIL5744, RAID1)

I'm trying to put together an eSATA RAID1 disk system for my new HR21. I've used an AMS DS-312 enclosure, 2 * Seagate DB35.3 750GB SATA 3Gb/s disks, and a 2m SIIG eSATA cable (though I've also tried the cable that came with the enclosure). The DS-312 uses a SIL 5744 chip, which is the same chip that is in the DS3RPRO that some folks have working. (So I thought this solution would have a good chance of working also.) I don't use any jumpers on the disks, so they are in 3Gb/s mode. The DS-312 also runs 3Gb/s on the eSATA port.

So, I restart the HR21, and disconnect power when the lights go out. I connect the eSATA and power up the DS-312. A short delay after the drive lights indicate they are up to speed, I reconnect power to the HR21. Then the "Almost there...a few seconds more" (yeah, right) screen comes up. During this screen, the DS-312's drive-activity LEDs light up 4 times, like this:

...about 5 seconds go by...
both drive activity LED's flash once (about .5 seconds long)
(a second later, I can hear the internal drive start up)
...another 5ish seconds go by...
both drive activity LED's flicker for about 1 second
...another 5ish seconds go by...
both drive activity LED's flash once (about .5 seconds long)
...another 5ish seconds go by...
both drive activity LED's flash once (about .5 seconds long)
(the internal drive starts making lots of seek noise)
(after a short delay, the screen switches to the "step one of two" screen)

I've tried a 2m SIIG cable and the 1m cable that came with the enclosure. Both seem to fit perfectly in the HR21 and DS-312, and both behave as described above. I've also tried reversing both cables, though both ends are identical.

I have connected the enclosure to my PC, though I can only do so with USB. :-( So, I cannot verify the eSATA is working this way. Ignoring that, the rest of the functionality works perfectly. The PC shows a 698GB RAID1 disk. I can partition it, format it, and check for errors (finding none). The PC likes it via USB.

(EDIT: bogus stuff about firmware deleted. The box already has the latest firmware for the DS-312.)

Does anyone have any idea what could be wrong? At this point, I'm thinking about moving to a DS3RPRO since someone has that one working, and it is almost the same feature-wise. It seems like the eSATA connection is working, due to the LED flashes, but there's no way to be sure. I wish the HR21 gave some feedback on what was going on...

Greg