Is it possible to use a little Raid 5 as the cache drive for unRaid?  Say 3 x 500gb drives I have laying around.

 

These would allow for some very fast writes no? And you'd have fault tolerance on that cache drive.

 

I guess the only way this could be done is to use a hardware raid card?

yes.. but you might find that 3 old 500GB drives might be be slower in raid then a new cache drive..

 

you would need a hardware raid controller that unRAID see's.

would it make sense to use a 10k rpm drive as a cache drive?

 

or how about mirorred 10k rpm drives?

 

EDIT:  Oh, I see there are even 15k rpm drives available these days. 

not worth the price.

 

if you raid three fairly modern 500's together, you would get at least gigabit speeds out of them. I'll assume your drives are sata.

 

If you get a controller with a BBU. you'll get better performance by turning on the write caching on the controller.