Can't change old PC HD over to newer PC??? Help

Easy job, but this ones got me... Changing over to a Compaq (d-510c evo) 2.4ghz P-4, from my old Compaq AMD T-bird 700mhz (mod.7970).

Thought I'd make my OCHD, (old computer hard drive), the [Slave] D:\ drive in the newer setup - 'til', I found most all my software, still on my 'older' drive, don't like and won't run from its new D:\ assigned location - no problem right! - I'll just swap the two HD's around (for now), so I can use my programs.

Then, the bomb! My original [Master] HD from the old PC (that worked fine before the switch), is now reporting from the newer PC boot screen, "No OS Found"... un-scrambled, I think that means BIOS can't find a MBR on that drive!?! "WHAT"-what,what,what? How can this be- it booted just fine as a C:\ drive an hour ago!

I ran a utility tester on it, and sure enough - 'No boot record found' on drive 01. Only the (original) HD @ 00 (in the Evo) will bootup? Then, it gets better-I've since (tested) 3 other good (used) HD's in the old (and new) PC and ALL of them are reporting, "No system drive found"

The only difference between the two PC's involved, (besides one being cir. 1997, and the other cir. 2003), is the newer (Evo) has Intel's Boot Agent utility setup on it.

Back when I used to do a lot more of this stuff, I recall this sort of oddity happening now and then - suddenly, every hard drive you'd hook up to a certain computer would start reporting, "Dead Drive" - but i don't have a clue what was causing it.

Help! I need a council circle here on this mess!
Thanks
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Setup: Both PC's & HD's were working Ok before. Both running win-XPpro (NTFS). Both HD's are under 100gigs. 3 of the 5 HD's involved were (known working) western digitals + 1 Fujitisu, 1 Samsung. Jumper settings were double cked. - all setup w/ XP, or 98se.