Hi Simon,
Thanks for the bug report, and sorry for the late reply. I'm travelling at the moment.
When you look at the drive in Finder (say, on the Desktop), does the name appear as "PHOTOS 1" or just "PHOTOS"? If you've connected two different drives with the same name, or the same drive in two different ways at the same time (such as via both SMB and AFP network sharing), one of the drives will have a number appended to its name in order to avoid naming conflicts. Normally I would expect System Events should be able to keep this straight for itself, but it is possible that it isn't.
A quick fix for your immediate needs would be a simple restart of your computer. That'll clear out the duplicate entries in the list of connected disks, so that your "PHOTOS" disk just appears as "PHOTOS" rather than "PHOTOS 1". In the long term, I'll look into reproducing this error and figuring out how to keep System Events from getting confused.
Thanks again for the bug report!
Jon