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A great deal has been written here about how to usins CF cards. I don't know if it's possible to boot off a SD card, but you can off CF, if you set your system up properly.
to those of you who are using compact flash cards as hdd how did you reduce the number of writes/erases or is it not really an issue? can you boot windows off a SD card? they have a much higher write/erase cycle but can they be used as a boot drive?