For the C64, C128 (in 40-column mode), and VIC-20, the BackBit cartridge is
still for sale at

https://www.backbit.io/

See Evie's introduction to the BackBit at

https://www.youtube.com/watch?list=PLik3CBZlL5eW3O-LaLSmVX5k3eL0nKWet&v=G3t2_il
oGVQ&feature=emb_logo

and how to use the BackBit at

https://www.youtube.com/watch?v=8YKruh_DiEY&list=PLik3CBZlL5eW3O-LaLSmVX5k3eL0n
KWet&index=2

The big selling point of the cartridge is its compatibility with various disk,
tape, and cartridge images, like D64, D81, CRT, PRG, P00, .20, .40, .60, .a0,
and more.  Just drop the image(s) onto a microSD card and pop the card into the
BackBit.  You see 
the file(s) on a menu, you select the one you want, and it loads and runs.  No
Commodore calisthenics to get the computer to recognize the image type.

As mentioned above, the BackBit cartridge works with the VIC-20 with a special
adapter that fits the BackBit into the adapter.  Evie has a video that explains
it at

https://www.youtube.com/watch?v=Une9mquKL0w&list=PLik3CBZlL5eW3O-LaLSmVX5k3eL0n
KWet&index=7

For an independent review of the BackBit, go to

https://enragedgeeks.blogspot.com/2020/01/normal-0-false-false-false-en-us-x-no
ne.html

          Writing from Sydney, Australia, at the moment,
          Robert Bernardo
          Fresno Commodore User Group - http://www.dickestel.com/fcug.htm
          April 25-26 Commodore Los Angeles Super Show -
          http://www.portcommodore.com/class

P.S. Evie Salomon is tentatively scheduled to appear at the April 25-26
Commodore Los Angeles Super Show.

--- SoupGate-Win32 v1.05