I know that this doesn't really help you, but I never use standby/hibernate mode. I have seen too many times that computers with it enabled have problems when resuming. Set power options to turn off the monitor and HDD but never standby or hibernate. As far as I'm concerned MS never got that right, sometimes it works sometimes it doesn't.
I have the exact same issue. My second drive is a WD 150GB. I too can see it in my computer before I hibernate but after it is gone and not in the device manager. I also have to restart to see it but as soon as I hibernate and awake it is gone.
Any help is appreciated
I'm running XP home edition
CPU AMD|A64 X2 3800+ AM2 2x512K R
MEM 1Gx2|CORSAIR TWIN2X2048-6400 R
MB ASUS M2N-E SLI NF500 SLI AM2
HD Primary 250G|WD 7K 16M SATA2 WD2500KS
HD Secondary Western Digital 150 GB PATA
This may sound a bit odd, but disconnect the SATA cable from both motherboard and hard drive, wait a couple of seconds the reconnect. This is kinda like re-seating a component and can clear up your problem.