On Sat, 2011-09-24 at 18:49 -0500, SpawnHappyJake wrote: > Also, constantly reading and writing to a thumb drive is hard on the > flash cells and will kill your thumb drive. I recommend copying it off > the thumb drive to a temporary location on your friend's computer, and > then copying it back on when you are done, and delete the temp folder. > I used to think that until I read up on wear levelling. All flash cards have a lifetime that's determined by two parameters: chip design life expressed as write cycles the design life of the package expressed as insert/remove cycles Cheap cards may be designed for as few as 10,000 write cycles while premium cards are typically designed for 100,000. Most card contacts are designed for 20,000 insertions and removals. The lifetime of an SD card is unaffected by the number of times it is read. Writing involves a chemical change to the chip: each time a bit in the chip is written it is degraded slightly. The material its made of is specified to last at least the design life before it degrades sufficiently to become unreliable. Calculations show that a good quality card is likely to be lost or trodden on before either the chip or the contacts wear out. This calculation assumed that the card was a 2GB SD card organised into 4K blocks and that it has a design life is 100,000 write cycles per memory cell and 20,000 insertion cycles for the contacts. These are typical figures for a premium quality card. The application, a real one involving logging GPS data as it is received, writes a 40 byte line to a log file once a second. If a premium SD card is used, it will take 303 thousand years before the cells in a memory block wears out. I also assumed that the card would be removed from the recording device and read into a PC every day (2 insertion/removal cycles per day). The card should withstand this for 27 years. As I said: unless the card is mistreated or faulty, you're likely to lose or damage it before it wears out. Don't believe me? The specs of a decent SD card, e.g. Crucial or SanDisk, are on the 'net, so download them, read up on how the cards are organised and how wear levelling works (both are well described in Wikipedia) and do the calculations for yourself. Martin
