Photos export in original format is just creating empty files
I am attempting to export my original RAW photo files from Photos to an external SSD. I make a selection and then File>Export>Export Unmodified Original Photos For XX Photos.., and then complete the pop up without ticking the Export IPTC as XMP and choose my folder on an external SSD (formatted ExFAT). However the resultant file, whilst professing to be the correctly named file (XXX.CR2) is in fact and empty file - zero bytes. If I export as the modified version then all works well. It is not the RAW format that is the problem as it will recur if I export an unmodified JPEG, and this problem only occurs when I attempt export to the SSD, it will work exporting unmodified files to the desktop which I can then drag to the SSD and subsequently delete, so I have a work around, but I can't see why this is happening. My software is all up to date Big Sur 11.5.1 and Photos 6.0. I have tried rebooting and and attempting it without any other programs running.
Any ideas
MacBook Pro 15″, macOS 10.13