The circle-slash usually doesn't indicate a permissions problem. More often, it indicates that you are trying to copy files from a Macintosh hard drive to a PC drive, and the Mac files are not able to be put on a PC drive--either because they are too big (a PC-formatted FAT32 drive is incapable of storing files larger than 2 GB) or because the Mac files have names that are forbidden on PC disks (PC disks can not have files whose names include certain characters).
Can you describe to us exactly what drives you are copying from and to? Is the drive you are copying to formatted for PC use?
I think you've hit on it. One of the drives is formatted as MS-DOS (FAT32) and I'm sure the other which was giving me problems is as well (it has already been put back in its' secure, non-disclosed off-site location).
So that's why it would accept 400 images files, but not a folder of them?
It's not a filename issue, I've learned how to avoid those by now.
Thanks for the answer.