I have 3 HP DX2400's with the exact same issue. When turned on, they only go as far as the HP boot screen that gives the option to run setup, select boot devices etc. No BIOS post screen. In all 3 machines, I've entered setup and the hard drive is not being recognized. When booting from removable disk, the disk will read, but no HDD is detected.
Are you trying with the same hard disk or a different one in all 3 systems? What's the hard disk model number? When you wrote "When booting from removable disk, the disk will read, but no HDD is detected.", did you mean that system still boots (presuming that the disk is bootable)?
If PC can post and boots of a CD/DVD, I would hook up the sata and power cable from the optical drive to the hdd as we know these are good, leave the sata cable plugged into motherboard, as we know this is a working sata port.
If that works, then start thinking about faulty SATA or power cable.
If it didn't work, faulty hdd or sata port or controller on motherboard.
All 3 systems have their own individual HDD, They are all the same model (160 Gb SATA) I don't have the specific model# because I'm not near the machines right now. Sorry for the confusion...no, the systems will not boot by removable media. A bootable cd/dvd is recognized and will run, however the hard disks will not boot because they're not being recognized. I have tried swapping SATA cables and ports, I've swapped hard drives from one machine to the other and the result is the same. I've contacted HP support and they've only offered me the suggestions i've already tried. Their conclusion is that the SATA controller has gone bad on all 3 boards. This is obviously not true because the optical drive is recognized on all 3 machines and these are SATA drives as well.