If you submit electronic files (e.g., digital photographs) on a CD to the US Copyright Office as part of a copyright application, the Copyright Office stores your CD but does not transfer the files on it to its computers or other durable media. There is also no way to resubmit or otherwise replace electronic files stored in the Copyright Office archives if the magnetic or optical media you submitted them on deteriorate.
This appears to mean that the registrations for many copyrighted photographs will become legally indefensible if/when the CDs on which the images were submitted deteriorate. The person with whom I spoke at the Copyright Office suggested submitting photographic prints or contact sheets rather than CDs. This suggestion would have been good advice until recently, but it’s impractical for people who copyright large numbers of digital photos.
I have no idea if the deteriorating-media issue will become a significant problem. Maybe not: the odds that any particular image file submitted in a copyright application will be needed to defend a copyright are low. Happily, the Copyright Office is testing a system that allows copyright registrants to upload image files over the Internet, and this new system should eliminate the CD issue for people who use it. But the many files that have been and will be submitted on CDs and DVDs are still vulnerable.