ps2 doesent read any backups...

[andex]

First of all i think u have duo 2 GT modchip. As u say u boot every single ps2 (original) without any problem but not backup copy. So why dont u try this way and see if that boot sequence work.
1. Press the reset button to power the ps2
2. Insert ur backup copy in.
3. Press and hold the reset button until it power off.
4. As soon as the power off press the reset button again and see if its work.

Im only try this boot sequence once. So give it a try...