A while ago I bought a Psion Series 3mx on eBay but found it didn't recognise SSDs inserted in either A or B slots and I suspected a motherboard fault. However, having found another fairly cheap 3mx to test this, and checking that it did indeed read SSDs OK, I tried swapping the motherboard but it didn't fix it. Not only that but it now failed to read SSDs when I reinstalled it back in the original machine.
I have now solved the mystery, and what to avoid doing when working with a Series 3mx, and eventually found and implemented a fix that solved the problem, which I'll explain below.
The only reference to the problem I'd found online was here: https://www.filesaveas.com/cgi-bin/forum/YaBB.pl?num=1123419681 which includes an old link to a solution which, frustratingly, no longer exists. There's no description of a solution to the problem but there is a hint that it was linked to microswitches that detect the state of the SSD slots.
This led me to look more closely at the motherboard and to compare it with a Series 3c one. Whereas the 3c uses a set of tracks and a small metal connector on the slot door hinge, the 3mx does indeed have a pair of microswitches, set at an angle.
Looking more closely at the ones in my 3mx I could see that the plastic ends of the microswitch button had snapped off, almost cetainly from prising up the motherboard with the SSD doors closed. The lesson here is clearly to make sure the SSD doors are open before attempting to remove the motherboard.
The switches do still press in but they'll need the buttons fixing. I have 3 out of the 4 broken sections so attempted a repair with superglue.
Re-gluing the microswitches was straightforward but very fiddly. Any glue on my fingers or tools attempting to join the tag onto the end of the switch would make it imposible to keep the pieces together while the glue set but, once it had, the result seemed pretty strong. Installing the switch was a bit easier, except for one where I manged to pop out the tiny spring that the switch pushes against. After losing it three times I eventually managed to get it back in and all switches working. For the missing one, I recreated the missing tab as best I could with a spare piece of ABS and that repaired as well as the other three.
I was sceptical about whether the repair would hold but so far they have done. I also found that it wasn't enough to repair one switch. It wasn't until I'd got both fixed and back in place that both SSDs were recognised.