My vote would be the addressing conflict.
The A500 is not an A500+, right? What is the make of this 2MB trapdoor expansion? It can't be mapped entirely as CHIP because the 512K on the motherboard begins that address space. 2MB is the maximum CHIP ram addressable on any classic Amiga.
I wonder if both the new A500 with 2 MB trapdoor AND your existing A500+ conflict with the address point of Supra's HDD controller. I don't recall if the 500XP is even compatible with the A500+.
Can you put a normal 512K trapdoor expansion in, map it as CHIP, and then put all the FAST ram (up to 8MB) that you require on the 500XP? I think this is the normal mapping of the 500XP.
