Hi,
I keep my photos on a LaCie external drive and have no problem opening my X100S raw files.
I just go to Lightroom and point to the appropriate catalog (which is stored on my internal drive with the backup on the external drive), and it pulls up all the images on whatever Drive name I have given the external. I use a Windows 10 system and can only assume that the Mac works the same way, but don't know for sure.
I just have to make sure I click on the Drive letter of the external in the Folders pane of Lightroom, otherwise it tends to pull up the internal drive letter automatically.
Hope this helps.
