I wanted to document some things I found out when trying to get Flash Floppy working with a 25 SX. It suffers like all PS/2s in that you need to run the diagnostic disk to reconfigure the system and the floppy drives can be unreliable. I considered buying a ps/2 to standard floppy board that are floating around , but decided to hack up an old floppy cable instead to try it out since there are lots of different approaches for different ps/2 models depending on whether you're using an edge connector and/or working with a 34 pin or 40 pin connector on the planar. Much of what I found before I tore into the floppy cable was about "old" vs "later" model PS/2s and how IBM was changing their floppy controller. This post was very helpful in ruling out the "older" 5 resistor pull up configuration for the cable:
Help IBM PS/2 model 8556 wiring 1.44MB floppy

It order to prepare the connection to the gotek, I decided to cut traces to ground on Pin 3 and carry power to the connector directly. Initially, I thought that was all that was necessary to make the gotek work with the PS/2. It seemed like less cabling and potential for shorts as the case is very tight in the 25 SX floppy bay. I would not recommend doing this. The grounded pins on my gotek were grounded on the top and bottom of the board which meant I needed to cut 4 traces (2 top and 2 bottom) and ultimately had to remove the floppy pin header and replaced it to finish the cuts. Severing wire 3 and pulling out 5V directly from the cable would be easier to do.

Based on the instructions in the above post, I cut pin 9 from the cable and set J5 closed on the gotek. J5 on the gotek grounds pin 4 which in the Flash Floppy docs states is necessary for some PS/2s (more PS/2 mysticism). This, apparently, has the effect of setting the drive in 2.88mb mode according to the boot order menu that I was able to access later from the diagnostic disk (more on that later). It was not clear to me why since according to the pdf in the other post, setting ground for pin 4 and pin 9 should set the drive to 1.44mb mode which did not work.

Now, I spent a lot of time troubleshooting another issue. 1.44mb vs 720K images. The diagnostic disk is 1.44mb and the DOS 5 install disks are 720K. HD vs DD is usually a non-issue in newer drives but sometimes one image would boot or not the other. After much fiddling with the FF.cfg file, and setting jumpers, I determined that no matter the interface setting (ibmpc vs ibmpc-hdout) or host setting (unspecified vs pc-dos) the 25 SX would only boot 1.44mb images if J5 (pin 4 grounded) and 720K images if J5 is open.

At this point, reading the "ps2_fdd_trm_s42g2194_00.pdf" from the other post, it occurred to me the signals are all active low. Rather than pull out the O-scope, I decided to set pin2 to ndens in the FF.cfg to complement the logic and now the gotek can boot 1.44mb and 720K images with no issue.

In all of my troubleshooting was never able to get rid of the 604 code on boot and this is because, I think, the drive is missing the media type logic sensing. Just hitting F1 will skip the IBM warning assuming there are no other errors you need to address.

In summary, if you have a "later" model ps/2 with the 40 pin board connector with a 34 pin 3.5 in floppy:
It is possible to use a regular 34 pin floppy cable.
Set the gotek drive at the final connector position after the twist. Mind the pinout on the planar board. The last 4 pins will be unused and carry voltages that could damage anything connected.
Cut pin 9 from the cable and leave disconnected
Sever pin 3 from ground connect to +5V to power the gotek.
Assume none of this will work when converting a real 3.5 inch floppy. I never tested that.

FF.cfg:
interface = ibmpc-hdout
host = unspecified
pin02 = ndens
pin34 = auto

Gotek jumpers:
J5 Closed (Ground pin 4)
S1 Closed (Drive select)

This was enough to configure the system to setup a CF to IDE device and get past the headache of PS/2 minutiae. There is something I would like to come back to (unlikely).
Knowing that the logic seems to be active low according to the pdf:

All input signals operate between +5 V dc and ground, with the following definitions: The inactive level is +2.0 V dc minimum Th […]
Show full quote

All input signals operate between +5 V dc and ground, with the
following definitions:
The inactive level is +2.0 V dc minimum
The active level is +0.8 V dc maximum.
All input signals are complementary metal-oxide semiconductor
(CMOS) compatible.

It seems that putting pin 9 and 4 high (active low) should set PS/2 to detect a 1.44mb drive properly:
(0,0) Non-secure 1.44MB 3.5-inch

Where as severing pin 9 sets it "high enough" and grounding pin 4 indicates a 2.88mb drive which Flash Floppy does not support but allows booting other floppy images.
(0,1) 2.88MB 3.5-inch Non-secure 2.88MB 3.5-inch

Perhaps using some different valued pull up resistors to ensure proper detection would help? My hardware knowledge has reached it's limits on this one for now.