Friday, December 11, 2009

Recovering Deleted JPEGs from a FAT File System - Part 1

Part 1 in a series of posts on recovering deleted JPEG files from a FAT file system.

In 2004, my in-laws returned from a vacation and had all of the pictures accidentally deleted from their digital camera. When I heard the news my first thought was that the data may still be there!

Often, deleting a file only erases its associated file system metadata (i.e. name, owner, etc) - the file's data is left unchanged. As a result, a deleted file can sometimes be recovered by finding the unchanged data fragments and recombining them in the correct order. I reasoned that if their camera took this metadata-only approach then there was a good chance that some of the pictures could be recovered.

Unfortunately, my in-laws lived in another state so I couldn't examine their camera's memory card right away. I suggested that they remove the card from the camera and bring it with them on their next visit so that I could analyze it then.

My initial plan was to try one of the many file recovery tools that already existed. However, I soon began to wonder how hard it would be to write a program to recover the pictures myself. Finding the challenge exciting, I immediately began spending all of my spare time coding up a recovery program.

After one week, and a lot of caffeine, I had a C program capable of recovering deleted JPEG files from a FAT file system - the typical file system used by digital cameras. Using my own camera as a test bed, the program could reliably recover pictures after performing an "erase all" operation. Cool!

Unfortunately, the story didn't end as well for my in-laws. When I finally received their memory card I found that all of the file system data blocks had the value 0xFF - a clear sign that the entire FLASH memory had been erased1. The data was gone.

Despite the unfortunate ending, this project was one of my favorite spare-time hacks. So much so in fact that I thought it would be fun to recreate it as a series of blog posts. Over the next few weeks I plan to write a series of posts under the label "FATrecover" describing how to develop such a program. By the end of the series, hopefully anyone with sufficient coding experience will be able to write their own FAT file system recovery program.


1 FLASH memory, being a form of EEPROM, cannot be re-written directly. Instead, the FLASH memory cells must first be returned to an unwritten state - an operation typically called an "erase". After being erased, the affected memory cells have a value of 0xFF.