I get the error E:Can't mount SDCARD. But... with the phone attached to my XP machine and using adb shell echo /dev/block/mmcblk0 > /sys/devices/platform/usb_mass_storage/lun0/file, I was able to then do cd sdcard at the # prompt. Once I did that I did mkdir test and it created a directory named test which I verified by disconnecting my phone from the PC, booting it to recovery console, and then doing Alt+X to get to the console. From there I did cd sdcard then ls. The directory named test was there. Now what?
Wait.... I think I had to do the echo /dev/block/mmcblk0 > /sys/devices/platform/usb_mass_storage/lun0/file in the console on the phone too and then I could see the test directory.