hi I don't know what base IOS versions I should use. I need to know whitch "base ios" to use for IOS 249 and 250. also are there other "base ios" that I need in other slots?
It all depends on the format of your usb hdd.
My usb hdd don't work with cIOS rev21, so I have cIOS38 rev17 installed to IOS249, and also use IOS223 and 224 to get games working.
You should get bases of 56, 57, and maybe 38. Cant see why you'd need more.