iMac randomly shutting down while copying photo library onto a external hard drive

I have a iMac 2014 & powered with macOS Mojave. I have been copied my photo library (just under 1TB) to an external drive but my iMac keeps shutting down. At some point I noticed that it shut down at the same moment as the external hard drive reached 276GB. I have used a different external hard drive, and iMac shut down randomly as well. I have also tried to export the unmodified photos to the external hard drive, but same outcome, iMac shutting down randomly.

No issue with synchronising time machine, which is also on an external hard drive.

Furthermore my iMac is set to "never sleep" or shut down" etc....

My iMac has always worked perfectly, never had an issue