How can I remove a folder from the itunes library

It depends on your iTunes preferences.

1.If iTunes is running default, iTunes actually copied all that media to your local drive, and you would have to delete it manually via sorting by "Date Added" to remove it.

2.If you have iTunes set to reference the files, when you disconnect the drive iTunes has some pretty annoying behavior where it will try to maintain the link but it will become a ghost music icon, the best way to get rid of these is sort by "Date Added" in your library and remove them manually.

These behaviors can be checked under iTunes preferences, I copied this pic from the Apple support site under HT1751: