This is an old post, preserved for reference.
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This may sound strange, but you can run a PET microSD IEEE-488 disk drive replacement on a VIC20. There is already the excellent SD2IEC SD card drive for VIC20 and Commodore 64, so why would you want to do that? One reason is speed, the IEC serial bus is quite slow in comparison to the IEEE-488 parallel bus.
kickstarter for a VIC-1112 replica, which should also work with this. The same should be true of a Commodore 64. I'm on the lookout for a C64 IEEE-488 cartridge to test it, but I do already have one PET microSD user running on an SX64.
Update: That cartridge is now in development, see The Penultimate Cartridge - Part 1.