my 106gb photos system library (yosemite 10.10.5 with icloud) needs to be relocated to my external USB drive (3tb). How do I do that?

First make sure the EHD is formatted OS X Extended (journaled) as shown in this screenshot and is locally connected by USB, Firewire, Thunderbolt, etc. Can't be wireless if you expect to operate the library from that drive.

Next, drag the library to the EHD and let the copying begin. Once the copying has completed launch Photos with the Option key held down and select the library on the EHD to open. Once you've confirmed that it's working correctly and all is well you can delete the library from your internal drive.