For Windows users, most AIR plugins will automatically install the VST2/VST3 file (.dll) to the following location:

C:\\Program Files\Steinberg\VSTPlugins

Make sure that the plugin manager in your DAW is directed toward this location in order to scan any AIR plugin instruments or effects. 

Note: the AAX version is Pro Tools only and will install to its own permanent location on the C Drive. This should not be moved. 

Can I move the .dll VST2/VST3 file?

It is possible to manually move the .dll file to another location on your C Drive. This should not cause any issues as long as you know where the file is located but could be problematic if the plugin is ever reinstalled without deleting the .dll file that was moved. Reinstalling a plugin will reinstate the .dll file in the location above. 

Can I move the .dll VST2/VST3 file to an external drive?

This generally is not necessary and not recommended. Unlike sample libraries and other audio files that can take up significant space on an internal drive, a .dll file is an active software file and does not require a lot of space. Moving it to an external drive may work, but could cause performance issues depending on the drive, drive speed, and quality of connection with the computer. Performance problems could occur in the form of crashing, signal delay, or poor audio quality.