Some classic 8-bit systems' floppy drives (notably: Apple, Atari, Commodore) did not have an index sensor and had only one read head. As a consequence, it was possible to flip a floppy over and use it to store additional data, a capability used by commercial software and home users alike. These were called "flippy floppies" or "flippies".
But! You can't read the flip side of these disks in an unmodified PC drive, even if you're using archival software like FluxEngine or GreaseWeazle. This guide shows a self-contained and reversible modification for TEAC FD55-GFR drives that enables you flip your floppies like it was the 80s again, and get both sides backed up.
Because PC drive electronics require the index pulse, the second side of flippy disks can't be read with unmodified PC drives (except for some extremely uncommon disks with two index holes). The second read head doesn't help, because the two heads are offset; the second read head's "track 0" is not the same as the first read head's.
However, it is possible to modify many floppy drives to supply an alternate revolution sensor, and then read the second side of flippy floppies! The method here is based on a procedure proposed by the author of fluxengine, David Given and is for TEAC FD55-GFR drives. The general principle can be applied to other drives, but these instructions are specific to the TEAC FD55-GFR. Even among TEAC FD55-GFR drives there is some variation as PCBs were revised over the drive's lifetime.
Our method is a little different than David Given's, because we made ours fully reversible. However, a consequence of the fully reversible version is that you have to turn off the new sensor using the switch whenever inserting a disk "right side up".
You'll find other methods proposed on the internet, including ones that make modifications to the metal frame of the disk to increase the range of motion and copy both disk sides with a single operation. If you're up to it, check out some other options and then do the one that is best for you! Otherwise, pick up the components below and let's get started soldering.