I don't know much about Atari. My only other retrocomputer is an RC2014 I soldered together. The Portfolio runs DIPDOS, which is compatible to MSDOS 2.11, but has fewer commands. Storage is extremely limited. Internal (usable) storage is 32kb, and I have a RAMcard with extra 32kb. I've looked into buying an extra romcard from ebay, but prices where 50€+ Afaik, neither the connector for the expansion modules, nor the slot for the memory card catched on, because a similar device, but with the first PCMCIA slot, was released within a month or so after the Portfolio came out.

There is a mod though, which would allow the use of CF cards. I still have to work up the courage to do it, because it involves replacing two logic chips, a direct wire from a processor pin to the card slot, the schematics are from a dead site I had to resurrect from the Internet Archive, and the instructions are in Czech.