The drives having the same series number (7200.10) does not mean "they're pretty much identical". The capacity increasing means the platter count may have changed (though the platter itself definitely has), which also means the total LBA count has increased. LBA count directly affects a BIOS's decision to use CHS vs. Large (ECHS) vs. LBA mode. That addressing mode choice also affects how a bootloader makes its decisions; i.e. when you write the boot blocks, it may have been when using CHS addressing, so it may have decided to write things to C=839, H=255, S=0, while on the new drive it may be using LBA which would use a totally different location. On the bright side, at least sector size didn't change.
I would suggest examining the addressing modes in the BIOS first, as that's the most likely explanation for a non-bootable PATA system (in your situation) that I can think of.--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.