Quick test for unrecoverable memory cards

There is a quick trick to determine if recovery is even worth trying.

1. Make a disk image of the memory card (a copy of the entire memory card in a file). It would be best if you did this as part of every recovery anyway.
2. ZIP (or RAR, whatever) the disk image file.
3. The size of the resulting archive file (ZIP file) roughly approximates the amount of data you can recover from the card. Nothing useful can be recovered if the size is something to the tune of 10 MB.

This applies to memory cards used to store photos or videos.

Do I need to make a disk image file?

Yes. Especially if you do not know what caused the data loss. If the memory card develops some kind of problem with the memory chips, the memory may continue degrading even while you are doing the recovery. Having the disk image file achieves three things

1. Eliminates the risk that the memory card fails completely,
2. makes the source stable against any possible creeping degradation, and
3. ensures no changes are made by software, including the filesystem driver.

What is it about ZIP?

The compression algorithms work by finding repeated patterns in data. In a most basic case, ABCDABCDABCD becomes ABCDx3. Most of the still image and video formats are already compressed. This data does not have repeated patterns and does not compress any further. In contrast, the long runs of empty space (zeros) compress to effectively nothing. Thus, if your 32GB disk image file is compressed into a 10 MB ZIP file, you know the disk image file contains only zeros. The first time I thought of this was during this discussion on Reddit. By the way, Sony cameras (and apparently Panasonic, too?) overwrite memory cards with zeros when the card is formatted in the camera (the discussion references Sony A6000, but that's true about most, if not all, Sony cameras).

Created Tuesday, January 15, 2019