Normal hard disk come pre formatted with FAT16 I think, its up to you to decide if you want to change it from that.
The sat boxes I have fiddle with all have a form of linux in flash memory built into the machine. If the linux see's a disk it might automatically mount it. Alot of boxes come with out the disk fitted but with everything else in place. They sometimes change the automount settings so it won't do it. They also don't give you the root password. If you load a live os via the usb port then go and edit the passwd file you can get rid of the root passwd and reboot it.
Then you su in and change the automount settings.
Once you fit the disk for 30 quid the box is the same as one thats 100 quid more expensive.
There are tons of websites out there to give you the options.
I won't give you links because I will get in trouble from Saab becuase most of them will also give details on hacking commercial channels/ones your not meant to see in certain countrys.