Scratch disk

@Mylenium scratch disks as externals are OK as long as they are SSD with fast connections (USB-C/Thunderbolt) and mounted prior to launching PS.

https://helpx.adobe.com/photoshop/using/scratch-disks-preferences.html

@defaultw973otfsnhfk This also points out the issue you have with the wrong drive format on Mac.

You'll need to reformat your drive depending on what OS you are using. ExFat is a Windows format.

https://support.apple.com/guide/disk-utility/file-system-formats-dsku19ed921c/mac

By their nature scratch disks have to be on permanently available drives, which can't include removable external drives or else you may lose data. I doubt that this will ever change.

Mylenium