Time Machine tells me to use it EVERY TIME I insert my external HD

"A workaround from Stop Time Machine from Nagging About Every External Disk at AFP548:

Every time you plug in a different external disk Time Machine asks if you want to use it for backups. [..] What we need is a way to set a policy that tells Time Machine to not ask about every disk that is plugged in. [..] Here it is:
defaults write com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool YES"

"I thought Hasaan Chop's solution would work for me, but I had to do this:

defaults write /Library/Preferences/com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool YES

Perhaps because I'm on Snow Leopard? Found on Apple Support."