These last years several expansions allows you to use mass storage (USB or sd...) on Amstrad CPC, each needing a ROM to use it correctly.
And if you use two of these expansions, you will need to load two ROMs, so less RAM for your Amstrad CPC.
So Offset did write UniDOS and drivers to be able to use another expansion, needing only one ROM with features being the same whatever the expansion. Unidos can manage actually :
Offset also wrote UniDOS Cartridge Creator (v1.4) which is an utility to create Amstrad Plus cartridges equipped with a patched firmware which allows to add UniDOS and its DOS nodes. You can furthermore configure up to 30 additional tool ROMs (such as Utopia, Maxam, Protext..) in the cartridges (compatible with ParaDOS and Burnin' Rubber) without requiring a real ROM board. The tool also let you automatically download the latest UniDOS ROMs from the official web site.