Even if the collar bomber had known his name was on the USB drive, it would have been very hard to remove it, according to Frank McClain, an independent computer forensics expert.

As computer geeks and investigators know, when users delete a file from a computer the file isn't deleted immediately from the hard drive. Instead, the computer takes note that the area of the disk where the file is stored is now available to be written over. So investigators can often recover at least snippets of data from files that are supposed to have been deleted.

With flash drives things are more complex, thanks to mechanisms built into the drives to prolong their lifespan. Because flash memory cells stop working after they've been overwritten too many times, flash devices use tricks called "wear leveling" to even out how the memory cells are used. A side effect of wear levelling is that it is "almost impossible" to completely erase data from a flash device, McClain said.

That can come in handy for people trying to recover photos or other files they've accidentally deleted, and there are many , , to help recover their data.

The collar bomber's first mistake was thinking he could delete something completely from his USB stick. But he also erred by not altering the metadata in his Word document. When Word saves a document, it automatically saves data, such as the user's login name, as part of the file. Office 2007 users can see this metadata by hitting the Office button, then "Prepare" and "Properties."