OK, In my new system I was having trouble driving 2x 3.5" drives from an OPUS. Basically the second drive was a USB 2 caddy with a 200 GB disk installed. The internal disk was a low profile 40GB HD.
I could never see the second drive after powering the PC up, only when I repowered the USB caddy afterwards. So here is my solution.
Use the +12v of the OPUS as the acc signal for an ITPS, the ITPS will wait 5 seconds before powering up the USB caddy. The +12v of the USB 2 caddy is powered directly from the ITPS which is fed from the battery. This way the caddy is controlled by the power sequencing of the OPUS/ITPS combination.
Well it works for me any how....
Audi S8 supercar
Modded Xbox 400GB HD
Rear 7" screen & navplus
Toyota Hiace Van
Shuttle PIII 866 256Mb 10GB OPUS powered
MediaCar & Mappoint 2004
Custom fit 7" TFT VGA TSK