Exit iTunes.
Make Sure the external drive is showing up as a volume in Finder
ReOpen iTunes.
The exclamation points indicate a problem - usually that the file is not found.
If reopening iTunes doesn't resolve, right-click one of the effected songs and select Get Info. Check the path listed there, and make sure it is still valid - it's possible that the external drive got a different volume name (maybe adding a 1 to the end of the name), making the path invalid.