Some kind of fast RAM configuration issue?

[Zac67]

Not only RAM, but *everything* on the Zorro2 bus needs to fit into the 8 MB address space. I am sure the GVP needs a few byes for SCSI I/O, so if you fill up the entire space with RAM, the SCSI does not fit in.

Not entirely true - there are two distinct AutoConfig spaces: $200000-$9FFFFF for large allocations (8 MB total) and $E90000-$EFFFFF (7x 64 KB) for small allocations (i.e. I/O). Any additional board failing to allocate memory space will just stay unconfigured/shut up.

So, in theory this should work. A possible problem might be that the two chunks get linked together and something rises from that. Have you tried adjusting the mask value of your partitions so the buffers don't end up in 32 bit RAM? Another approach may be to limit the GVP to 4 MB and AddMem the last 2 MB manually after booting (depending on the board's hardware this may not work).

[Zac67]

The mask values need to be changed in HDToolBox. You need to use e.g. ShowConfig to find out which RAM maps where.

If for instance the GVP maps from 0x200000-0x7FFFFF you'd use a mask value of 0x7FFFFC to force the buffers to 16 bit RAM.
If the 2630 maps from 0x200000-0x3FFFFF it's a bit more difficult as the masking scheme isn't really designed for this... 0xDFFFFC could do the trick.