I have a 2.5″ 250 GB IDE hard drive to form the basis of my rebuilt SlimNAS server. I also have the HP T5700 host platform. As week or so back I ordered a 40 pin IDE cable on Ebay just to see if the drive could be plugged into the IDE header in the T5700 motherboard, where the DOM normally lives.
When the cable arrived, from Taiwan no less, I tried this and found it works fine. The IDE header on the mobo is not keyed like on larger systems so it’s possible connect it in reverse. If you do this a couple of the leads heat up really quickly. Luckily I noticed this and powered it off quickly. Not damage done luckily.