I formatted one of my external hard drive as I'm planning to sell it. I used the regular (non-quick) option.
So is the data that was previously on the hard drive irrecoverable? Or is there some program I must use that can do this. I ask this because 'I think' I remember reading about other programs that can recover data even from a full format. But I never messed with stuff like that.
It is possible to recover from a simple format. If you want to wipe the drive to a point where it is not possible for the data to be recovered you need software to do it. There are even several ways to do this from a low level format which writes to every sector of the drive
Because some people claim that even after wiping the drive it is still possible to pull data off with recovery tools. If you were to wipe writing all 1's or all 0's to the disk it may be possible. If it is using a good pseudo-random generator for the writes it is highly unlikely that it will be recovered.