Is it possible to recover an overwritten video?
A common problem with lost data is when a file has been deleted, or reformatted, and then more video added to the memory chip. What can be recovered?
When a file is deleted what happens is that the pointers to the file are marked as deleted and the file allocation table (FAT) is cleared down. This means that for a FAT and exFAT file system details of where the file started may still be discovered. When a new file is written, the camera will typically overwrite the directory entry and typically start filling the data from the first free area in the FAT.
Example
The simple effect of the above is probably easiest descibed by the following example.
A 32GB chip has 16GB of data on it when it is formatted/deleted. If someone then writes 4GB of video to the chip the new result will be, 4GB of new video, 12GB of original video, and 16GB of used chip. Thus in this example 12GB of data could be recovered.
If however if the new video was 24GB, all the original data would be lost.
Can overwritten sectors be recovered
Much has been written about recovering data from overwritten sectors on a conventional hard drive. Examples of this being done on real life are limited, and accuracy also limited. Since about the year 2000, recording techniques have made this type of recovery impossible . Unfortunately, the myths live on, and there are suggestions that a disk must be overwitten 7 times before safe. As something to think about, if overwritten sectors could be recovered, the manufacturer would use this feature to double the capacity of the memory chip!
With a solid state memory chip, once overwritten, the data can not be recovered - even with the budget of the FBI.
The bottom line of the above is if videos are deleted, do not allow anything to be written to the memory chip. This includes any recovery program, or any undelete program. GPR software has built in safety features to prevent the origional memory chip being changed by the software, ie it will never save data back to the same memory chip - even if asked to!
Types of format
Many computers/cameras allow for two types of format. A quick format is the default version. This clears down all directories and file allocation tables. Because data sectors are not overwritten, recovery is possible. A quick format takes less than 30 seconds
A full format, which can take an hour or so, overwrites every sector. Nothing can be recovered from this type of format.
GPR software will recover from the last writing of the memory chip, and the forensic version can often extract meaningful video from the partial data that has not been overwritten. However, overwritten sectors will be remain lost forever.