If you mean the SD3 Core Library, yes! You can even define the “backup” to SD3 as an alternative path. I copied this out of the manual;
Add Alternative Library Path:
This allows you to set up a secondary path for the desired sound library. This should only be used if you have 2 separate installations of the sound library in 2 different locations.
For example, this option would be used if the full sound library is installed on a large external drive, but a smaller version of the sound library is installed on the computers internal drive. In this example the primary path would link to the full installation on the external drive, and the Alternative path would link to the small installation on the internal drive. Setting up your paths in this way would ensure that Superior was always able to load a version of the library even if your external drive was not connected.

Hope this is helpful.

@cjphoto said:
Thanks for your response.

It brings up another issue- is there a way to “copy” the first download to another internal or external drive without a second lengthy download process, using carbon copy pro or the mac finder? Im just trying to get around the lengthy download process.  

Yes, it’s as easy as finding the folder; SL-SuperiorDrummer3 and copying (entirely) it to another disk.
I have moved mine a long time ago, so take this bit with a grain of salt but it may be located in; /Library/Application Support/Toontrack/Superior Drummer 3. If not just use Finder to find it and copy it to another location.