I have a problem specifically with Photoshop Elements 12 Organiser and the ability to write to the internal drive on an iMac 5k with El Capitan. I have full read write access to the files on the disk but when I try to write the metadata to the photos I get the error "An error occurred while reading files or writing files to disc. The disc may be full or there may be a problem with the source media". I also have other issues accessing and erasing from SD cards.
What happens if you save your Photoshop file to a different directory or hard disk?
What are the file Permissions? Select your file in the Finder and go to File > Show Info > Permissions
It sounds like your file or folder permission are messed up. Also, Repair Disk Permissions (Applications → Utilities → Disk Utility):
I have specifically added myself as a user with read/write access. I had assumed that the 'everybody' group with read/write access would react in the same way as being named separately.