and put them onto a ExFat volume
Does it need to be an EXFat volume? Having the referenced files on a disk with a different formatting than your Aperture libray is asking for trouble because the filenames may be incompatible. If you can, move your originals to a drive that is formatted MacOS X Extended (Journaled).
But Aperture should be able to see the drive and the volumes, even if it is ExFAT. I just checked, my Aperture 3.5 can see folders on ExFAT volumes, at least, after I enabled the "Shared Folder" flag. Check the permissions on your drive, Phil. What does it say?
It's just as I didn't want to restore them onto a volume that I was restoring them from, the minimise overwriting freespace that may contain more images.
I found that holding option on a root folder allows Aperture to force relocate.
But still I'm thinking I may need some kind of script to make my Aperture image previews into the masters.
I will report back as to how many images I recover.