Here's something that might help you identify the "unreadable" files:
You'll need CarbonCopyCloner.
It's FREE to download, and FREE to use for the first 30 days.
Get it here:
http://www.bombich.com/download.html
Now, you need to use CCC to "clone over" the photos to the external drive.
BE AWARE that you will have to "set up" CCC so that it will clone ONLY the photo files involved.
You can do this in CCC's window on the lower-left, by UN-checking things that you don't want copied.
There will be A LOT of things to un-check, but take the time to do it.
NOW...
Let CCC do the backup of the photo files.
As CCC copies the files, IF it encounters a "bad" or "un-copiable" file, it will make a note of it, and then continue on with the backup.
At the completion of the process, CCC should be able to present you with a list of those files that were not copied.
You can then use this to check the "bad" files, perhaps open them (on the original drive, if possible) and tweak them to a point where you can create a "new copy", etc.
WHY I suggest the above procedure:
As mentioned above, I believe CCC will IDENTIFY each un-copiable file.
BUT... if you do a simple "finder copy", the finder may tell you that "some files weren't copied", but WILL NOT "identify" them for you.
If you try this, let us know if it worked for you…