File Backup - Can't stop Sleep?

Drag copying files from external hard drive to a newer external drive. Copying large photo archives estimated to take about 3hrs to complete. Using a Macbook Pro (power cable attached) I set "Computer Sleep" to never. "Hard Drive Sleep" is NOT selected. Display sleep set to about .5hr. I returned to find Macbook sleeping (required password login to access) One external Hard Drive (destination drive) had also entered "sleep". I was prompted that hard drive was not ejected properly etc. I also got an Error Code -36 stating that an image file could not be read or written. Permission on this file indicates Read & Write OK for anyone.

I was drag copying as it seemed a more direct option than configuring time machine or carbon copy cloner.

I suspect the failure to copy the specifice file might be a separate issue from the hard drive & computer sleeping. How can I prevent sleep to copy large quantities of files. (photo archives) Any related suggestions here?

The external hard drives are the same brand, recent drives (one new) and both formatted as Mac OS Extended (journaled). Neither is designated as a boot drive.

Thanks for your help