I designed such an Item in 1983.Did anyone ever make a disk drive for the ZX81?
Based on 3" and another with 3.5" Floppy
It worked very properly and was able to make data transfert between ZX81and ZX spectrum.
Roughly one thousand pieces had been pruduced
Due to change in the company strategy we stopped this small production in 1986.
It was part of a full system called VP100 who included Display keyboard 64 k Memory board etc.....
The Disk drive system uses conventional components easy to procure.
I think I still have schematics and PCB prints somewhere
If somebody have an interest, please feel free to ask.