I've set up several instances of Raspian to boot off of the SD but run EVERYTHING after boot, including the OS, off of a USB drive. If I figure it out with Arch I'll post it.
IMHO, that's the way to go, not just move the file sharing directories. A lot of people don't seem to realize it, but even USB 2.0 is quite a bit faster than accessing the SD, if you have a USB drive it just makes no sense NOT to run the OS off of it, you'll notice the difference in speed. Also, USB drives are in general designed more robustly, they are less likely to get corrupted files if there's a power problem or something.
Unfortunately, the last time I checked there was a bug in Raspian that made it impossible to reliably boot off of a certain USB drive IF there was more than one USB drive attached. Not a problem with just one, though. In one instance we worked through it by using the GPIO to turn power on to the second drive... maybe it's fixed in the OS now.
IMHO, that's the way to go, not just move the file sharing directories. A lot of people don't seem to realize it, but even USB 2.0 is quite a bit faster than accessing the SD, if you have a USB drive it just makes no sense NOT to run the OS off of it, you'll notice the difference in speed. Also, USB drives are in general designed more robustly, they are less likely to get corrupted files if there's a power problem or something.
Unfortunately, the last time I checked there was a bug in Raspian that made it impossible to reliably boot off of a certain USB drive IF there was more than one USB drive attached. Not a problem with just one, though. In one instance we worked through it by using the GPIO to turn power on to the second drive... maybe it's fixed in the OS now.