try right clicking on an existing source and look for a delete option to delete a source you want to remove. What operating system does your device use?
Well, the device is probably more like an Nvidia Shield than a NUC. I believe it's Linux-based and runs OSMC and/or Kodi. I was finally able to locate the USB drive in the relevant dialog box in a folder titled "media" and updated my library. I'm new to Kodi, and some of this has been counter-intuitive, at least for me. Hopefully, I'll catch on.