Watched folder does not work on External USB Drive.

I am running Mac OS X 10.5.7. When i have the watched folder in my home directory, for instance in the standard folder music, songbird detects changes in the folder just fine.
If the watched folder is on the hard drive connected to my Airport Extreme, songbird detects changes in that folder just fine, BUT if the folder is on an external USB drive connected to an USB port on the mac itself there is no reaction when new songs are added to the "watched folder". Wierd...

So the problem is just that on mac the watched folder do not work proberly on an external USB drive connected directly to the computer.