Updating Photos Library stalled at 5%

I had moved my photos library from system drive to an external drive due to its size, about 400 GB.

4 TB external, formatted OS Extended (Journaled). This under Yosemite and Photos worked okay with the library on the external drive, even though a USB 2 connection.

I just updated to High Sierra 10.13.6 and when I started photos, immediately the Updating Library screen came up and it has been stuck at 5% for a few hours. My fear is that it created a Photos library on my internal drive and is copying back my photos from the external drive. I had assumed it would use the previous settings from Yosemite for Photos which has the external drive as my Photos library. If this is happening, how can I interrupt the program without damaging anything?

Mac Mini mid 2011 2.7 GHz i core 7.