As I can't get the DIN14 plug to make my GOTEK an external drive, I have not tested presented solution, but I believe that there is a way to swap A: and B: without (well, almost) cutting and/or bending any pins. It has two variants (STe only).
* First variant is to install ID switch (1P2T) in the Atari, and in the external drive. ID0 for EPSON SMD380 is by shorting pins 14 & 15, while ID1 is by shorting pins 15 and 16. You have to enable DS0 signal for external floppy by cutting a bridge between pins 2 & 3 of W300 and connecting pin 2 of W300 with pin 1 of W301. The drawback is that you have to move 2 independent switches to swap drives A and B.
* Second variant requires only one switch in the Atari. It has to be a two circuit ON-ON switch (2P2T). One circuit switches the internal drive by shorting jumpers 14 or 16 with 15. Second circuit switches pin 2 of W301 between 1 (DS0) and 3 (DS1), effectively switching external drive between ID0 and ID1. In this case you have to cut bridge between pins 2 & 3 of W301.
Be careful and think what you're doing - it has to be a cross-switch:
- in one position internal drive is ID0 (SMD380 pins 14 & 15 shorted) and DS1 signal is routed to the external drive (W301 pins 2 & 3 shorted),
- in second switch position it should be opposite: internal FDD is ID1 (SMD380 pins 16 & 15 shorted), and DS0 (W301 pins 2 & 1 shorted) is routed to the external FDD.
Pin 1 of W30x is a square pad.
I have noticed that D0SEL signal from YM2149 also goes to the U307 (74LS244) of DMA port to create BD0SEL. If one would swap signals by YM pins 19 & 20, then effectively D1SEL would create the BD0SEL signal (which I can't find where it goes). I'm not sure which method is better (swapping signals directly by YM2149 pins or switching FDDs between ID0 and ID1).