There's been a lot of talk about this. You're not defragging the BB, you MIGHT be defragging the memory, but it's not like defragging a PC's hard drive. I know people will argue and say "but I can see the drive on my computer", I assure you, you're not defragging and you didn't notice a difference.
When the BB shows up as a harddrive, you are seeing the SD card in your BB. This is a flash drive with relatively small capacity (not hundreds of GB) and containing many small files. Defreagmentation of a drive will not do you any good as a) its a flash drive so its rather quick, b) small files will not be fragmented as much as large files so you will just naturally not cause as much fragmentation. If anything you are shortening the life of your SD card as with any flash memory, there is a maximum number of writes before the memory burns out. Don't defrag your BB on your PC.
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thanks for the advice. When I did do it and said it seemed faster, I meant the picture shufflinf seemed faster. I have an 8g memory card and a lot of times, shuffling is very laggy. The defrag sped it up considerably