I was surfin the net today and came across some juicy info on Sky+ (the equivalent of MySky in the UK).
There is a secret installer's menu available by doing the following:
I don't have mysky to test myself, but the sky+ and mysky STB's are physically the same, hopefully the firmware is similar too so this will work.
[Moderator edit: removed step by step instructions]
source http://www.eeggs.com/items/30908.html
more info: http://www.chrisgreen.co.uk/hacking-a-sky-box/
http://www.pycs.net/users/0000348/weblog/2004/08/
http://www.radioandtelly.co.uk/skyplus.html
includes info on upgrading the HDD.