I can't help you with such a niche computer. However a quick search seems to indicate you can inject files to an already formatted diskette using any PC AT machine.
That's the only lead I could give you. When I transfer files to IBM PCs in MAME I burn them to CD-ROM images or use tools like IsoBuster to write directly to .img files. You could also write to a raw hard drive image and convert it with chdman or even transfer using communication ports or the Internet. Yet I haven't bothered with them because while effective requires a higher level of experiment and learning.
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Re: DEC Rainbow 100
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Re: DEC Rainbow 100
