It's the wrong approach if you just want a prosthetic memory to help people remember stuff.
To have a prosthetic memory what you need is a computer that can remember stuff - video, audio, photos, text etc. Preferably wearable. Then what you need is to attach a device to appropriate parts of your brain that reads thought patterns that are distinctive depending on what you are thinking (elephants, purple etc). The device does NOT have to decipher or understand what you are thinking. All it needs to do is associate the stuff to be stored/recalled or even _commands_ with the thought pattern(s) you choose for it. I call these thought macros. See also: http://slashdot.org/comments.pl?sid=3478821&cid=42956909
So you capture a video/audio/picture then you assign it a thought, or "current state" of mind. If you even have difficulty rethinking a thought pattern, you could search by context and time (what I stored some time ago while at home).
There may need to be training phases like in speech recognition, and it's likely to work better with some people than others.
 The approach the military is taking would still have problems if people can't even remember that they are supposed to remember something- so whichever approach you'd need the ability to set up "prompts" based on time and context (and brain patterns).
I believe our technology is very very far from the state where you can drop in a memory device with memories already preloaded in, and which people can use to "remember that they are to remember something" (and even if we did, it would be scary and I won't want to have it).
Because there's evidence that memories are stored differently on different people's brains - some people have a halle berry neuron: http://www.caltech.edu/content/single-cell-recognition-halle-berry-brain-cell
Seems to me to be a bit like a Bingo hall where a neuron yells bingo when it recognizes what the "announcer reads out". And the thing is those neurons aren't in the same place for everyone, they might not even be present for everyone, and one neuron might yell bingo for slightly different things (in one person they might have a neuron that goes bingo for Jennifer Aniston when it sees Jennifer Aniston + Brad Pitt, in another person it might not go bingo for the couple).
Which is also why I think that it's delusional for people to believe we'd soon be able to transfer our minds to other machines. You can transfer something, but it'll be far from everything.