If you added an entire drive (or a very large folder) as a VST folder on the Advanced tab in Reason's Preferences, resulting in that Reason doesn't launch (or stops responding), proceed as follows:
Quit Reason and then delete the "Reason Plugin Folders.prf" file in
The user (~) Library folder is hidden by default. To open it, press the Option key while opening the "Go" menu in Finder, then select "Library".
Then, launch Reason. Now, only the default VST Plugin Folders should be scanned.