Your MBPro is designed to go to sleep when a magnet is moved into the location where the sleep sensor exists.
Completely normal, although that is USUALLY what you want to happen when you close the lid.
So, you might not expect it to happen, so just be aware of the location of your external drive --- or find a different location to place your drive. An inch or two difference should be enough for the internal sleep sensor to either "sleep", or not.
Unless you prefer to sleep your MBPro with that technique, rather than closing the lid.
(to answer your question directly --- does not damage anything)