Something like this happened to me this morning after having my computer off overnite. When I booted up this morning, I could see the 323 drives still mapped in WinXP but couldn't access them. The 323 did not show in Easy Search. The message I get from WinXP was drives were disconnected. I have wd caviar green drives installed in the 323 and I have hibernation set on the 323. I am using the latest beta fw for the 323 (1.08).
I rebooted the 323 and eventually the drives were accessible.
The caviar green drives, if inactive, will normally spin down on their own by design to save power. My theory at this point is that when hibernation is set on the 323 and the caviar drives spin down on their own, for some reason it later becomes impossible to reconnect to the drives when both hibernations kick in without a reboot on the 323.

I have disabled hibernation on the 323 to see if this makes a difference.