There are different levels of fast. You need to use a fast one like Class 8
or 10. I run my netbook on a 16 GB Class 8 SD card and it is fast enough. My
home folder is on the 160 GB HD. I have run it on a no name usb key and it
is usable, but not great. I prefer the SD card to usb key because it is
harder to dislodge.