My external SSDs do not show up in Preferences > Scratch Discs

[LMNO P.]

I have an M1 Mac. Only the on-board SSD shows up in the Scratch Disks menu. The SSDs work perfect on multiple computer. Doesn't matter which cables I use either. Any idea how to get Photoshop to recognize the external SSDs?

[Eugenio.NYC]

it must be formatted as Mac OS Extended (Journaled) 

Go to your Application folder and open the Utilities and reformat your disk

1- open disk utility app

2- select the new disk (on your left) and click erase on top right

3 select the format Mac OS Extended (Journaled)  and erase 

LMK

[J453]

Make sure Photoshop has Full Disk access in the System Preferences: