smef@n:
please take some time to carafully read the documentation..
SME (if you DON'T use --noraid flag at setup time) will always create a Raid1 array even if with only one HD..
so, all you have to do (apart reading the documentation, of course) is add the second hd and add it to the array via the console
about hot-swap: I have 2 ML310G4 with hot-swap hd.. but I always prefer to shut-down the server

in any case, if you do an hot-add hd, let me know, thank you