How do you get erased data back on ipod touch?
If you use iTunes then your Ipod should have backed up all your applications onto your account. Therefore when you are next on your iTunes add your back up data to your ipod.
If i back up my ipod touch and then restore the back up will all the saved data of the games stay?
Unfortunately no. But a lot of games that require an account usually keep your progress on their servers so when you restore it should still be there.
Erased data, can i get it back (from an external harddrive)?
yes you canmost newer recovery software will scan and recover from even removable hard drives/ flash memory.. etc
How do you back up save data from games downloaded from Installous on an Ipod touch so that you can restore it?
theres an app called app backup you have to do research on it because my dad hacked mine ipod