Please be forgiving if I've completely got this wrong....
I've done some testing with recovery programs like Rescue Pro - it looks like overwriting the whole of a USB drive several times generally clears whatever is on it, though I'm not sure if this is just because the odds are in favour of the USB's wear-levelling allowing enough of the old files to be overwritten to make them undetectable. This isn't necessarily the same as overwriting the whole drive and definitely destroying all remnants of the previous data. I’m not clever enough to check this in more detail, but as I understand it there is always a possibility someone may be able to recover data fragments from an “erased” USB stick because wear levelling means it is unlikely absolutely all areas of a stick get to be overwritten when erase programs are run on it, or the stick is reformatted. I've spent ages trying to find a program that guarantees it will securely overwrite an entire USB stick, but the problem seems to be wear-levelling means this cannot be guaranteed, and of course even if someone does guarantee it, who knows….
I use USBs to move data between computers, and it would be a pain to have to work around this. So I’ve hit on this way of being sure everything on my USBs gets overwritten at least once, which as I understand USB memory should be enough to thwart most attempts at recovery.
1. Use truecrypt to create an encrypted volume on your hard drive as close as possible to the size of the USB stick (or perhaps 1,2,3 etc GBs).
2. Change the suffix on the encrypted file’s container to .iso, if it isn’t this already.
3. (This step is probably only for the paranoid...). Use truecrypt to mount the encrypted .iso file, so you have a truecrypt volume which should show in explorer as entirely empty. Use eraser (or any other erasing program) to securely erase the free space on the encrypted volume by writing random data to it. Dismount the encrypted volume.
4. The encrypted volume is now simply an iso file full of completely random data. Copy it to the usb drive. Copy it again and again to the USB drive (renaming it each time) until no more copies of the iso file will fit (eg if the USB stick is 4GB, you will be able to copy a 1GB iso file only 3 times).
5. Use windows explorer to ascertain the exact amount of free space left on the USB drive.
6. Create another encrypted volume the exact size of the free space, or as close to it as possible, set the suffix to iso, (mount it, fill it with random data, dismount it), copy it to the usb.
7. Use explorer again to see if there is any free space left on the USB. If there is, repeat step 6 or copy a small file of rubbish data to it or (assuming the space is negligible) use eraser to write random data to it several times so that even with wear levelling every memory cell left gets overwritten.
8. With the USB stick as full as possible with encrypted iso files, and any titbits of storage space filled with junk data or otherwise erased, delete the iso files from the usb using explorer (or if you are still paranoid, use eraser to “overwrite” them, although they may not be fully overwritten…..). You can keep the iso file(s) on your hard drive for future use.
Job done. Not as quick as using erase software alone, but I think safer and more certain.
Would welcome any comments on whether or not this is a sure method of erasing an entire USB stick. The advantage I see is that you know for sure the entire drive, all but perhaps a few bits, gets completely overwritten.
I
