SDX Plus is an upgrade for an MTX with the later variety of SDX (the kind with a single 3.5" type 07 drive, and possibly 512KB RAM).
The upgrade comprises a couple of EPROMs and GALs, no soldering is required and the process is reversable.
BDOS Err R/Obug
320KB RAM Disc and virtual tape support is likely to be more useful than having 512KB RAM Disc and no virtual tape support.
Having 512KB on the SDX and using 64KB as main memory, 320KB as RAM Disc and 128KB as virtual tape, is a better story than having no memory on the SDX and only making use of the memory on an MTX512 or MTX512 S2 motherboard.
These changes may be of interest to other SDX owners, and maybe owners of "Memotech Business/2" systems.