I've had the same problem with some HP and DELL computers with more than 2GB RAM.
I could be helpful to know which HP PC's makes the trouble ...
Sometimes it helps to change some BIOS settings, sometimes I unplug one RAM module...
BIOS settings for DELL:
Drives - SATA Operation:
RAID Autodetect /ATA
Post Behavior - OS Install
For HP I made a BIOS update and than I can change the SATA settings too
Take a look here: OSD Troubleshooting Guide
Hello, i also had the idea that SATA could be invoked somehow...
The used modells are DX2000, DC5700 and DC5750.
I´ll try to update the bios and change SATA-Settings...
I had a similar experience with the DC5700. I had the drivers loaded correctly but WinPE would not recognize the HDD. It turned out to be an issue with card reader. On the DC5700 (and probably other models as well) there is an optional card reader that comes with the machine. WinPE tries to boot to the card reader. My solution was to unplug the card reader.