I have an AMD phenom ii x2 550BE that boots/runs just fine when bios is EC=normal ... on two different boards (gigabyte MA785GM-US2H and gigabyte 790XTA-UD4) ... I also have a phenom ii x4 965BE that boots/runs fine in either board regardless of EC being Normal or Hybrid.
with the 550BE in either board, if the EC is set to Hybrid (even with the core 2 and core 3 both disabled) the system will not load the OS ... it makes it past POST and announces itself in the initial start screen as a X4 B50 (as expected) but hangs during the OS load.
on the MA785GM-US2H with EC=hybrid and core 2 and core 3 disabled, I did get it to boot, but the OS (Ubuntu) only shows 1 CPU in the system ... when my 965BE is in the system with those settings, it shows 2 CPU's as would be expected.
Hmmm I can only assume that there could be power issues?
Either that or the cores are bad and that is the reason. Not every X2 or X3 can have the cores unlocked. They were locked for a reason, mainly because they tested bad.
With my 720 BE, I got lucky. I've been running for over a month now with no problems. I've even overclocked it and still no issues. It's basically a crap shoot on whether or not you can unlock the locked cores and get a stable unlock.
From what I understand, the X2s supposedly have a higher unlocking rate than the X3s, but still, not every one will unlock the cores and run.
When unlocking my 710 on a friends sb750 mobo, it reacted the same way until I increased the volts. The stock was 1.325v in the bios, and it would boot and pass the limited stress testing I could do at the time, by upping it to 1.4v, iirc. I didn't have a lot of time for testing as it was just for a few hours to screw around with.
So yeah, maybe it needs voltage, maybe it just has actually bad cores.