Blank JPEGs

wrote:
As a backup I moved a lot of images onto a DVD and, for safety, also
to an external harddrive. They were all Jpegs from the same camera.

So far so good. Backups to two independent media types should mean that
both copies have to fail before you have lost data. This provides a
test. Find an image lost on one but not on the other and then use a
binary compare to see what changed. Bitrot on badly stored (hot, sunny,
humid) or low quality DVD seems the most likely candidate. I don't trust
DVDs for archive storage of my own images and I only trust some brands
of CD.

Now copying them back onto my PC several of them show just a blank
square with a tiny red cross in the center and the message ‘No Preview
Available’. When I click on Properties it tells me that this
particular blank is a JPEG of 3.95MB. But whatever program I try to
open it with, the message is always the same ‘unknown file format’.

Try opening it in a web browser. They have the most tolerant JPEG
decoders of any consumer products. Avoid Photoshop - it will fail on
JPEGs with even the most trivial or superficial damage.

Such affected images were identical on both, DVD and external drive.

This is *very* bad news and suggests that the images were already
corrupted before you archived them from your PC.

There are some malevolent viruses that delete or wreck JPEGs, but they
tend to get them all. Not seen a selective case before.

I would suggest you do a virus scan though. And run ScanDisk to check
for errors and then SMART diagnostics on your hard drive (backing up
anything else precious first as insurance).

Does anyone know what happened and how to revert them to the original
image?

Something hostile or a random hardware failure has altered the header
enough to prevent them from being decoded. They might be fixable but
don't hold your breath. The first thing to test is whether they are
anything like a valid JPEG stream. I have a free command line utility on
my website called SFILE which measures the bytewise entropy of a file a
good JPEG should come out at 5.4, text is ~2.5 and a blank file is 0.

http://www.nezumi.demon.co.uk/modula/m2.htm

If you don't like running binaries from an unknown source then use any
binary compare against NUL or a debugger to dump the first few hundred
bytes and either post them here or send them to me privately.

I am interested in corrupted JPEG images. If the failing file has an
entropy of 5.0 or higher then there is a chance it can be fixed. If you
send me one privately I can quickly determine if there is any hope of
image recovery. If the entropy of the file is 4 or less then the image
is long gone.

Regards,
Martin Brown
(my unlikely looking email address is valid without modification)