Backing up to external USB drive

[BLOND37]

trying really hard not to lose my cool over this.. have a MBP running lion blah blah got all updates blah blah.. had time machine running great then i did something and messed it up.. now all it does is wants to back up the back up drive to itself and won't let me change it. i want to back up my mac HD to the external usb drive like i had it before.

 

[Ccrew]

trying really hard not to lose my cool over this.. have a MBP running lion blah blah got all updates blah blah.. had time machine running great then i did something and messed it up.. now all it does is wants to back up the back up drive to itself and won't let me change it. i want to back up my mac HD to the external usb drive like i had it before.

Make sure the external is formatted HFS+, Select Preferences in Time Machine and point it to the external drive. If that doesn't work try less blah blah and more specifics so people can help.