Back-up of 563 GB iPhoto Library to external drive failed last night, plus external drives inadvertently ejected

I'm still in El Capitan (iMac 27" Late 2014) and have to upgrade to Mojave ASAP. I never used Photos because I needed iPhoto's places feature and was afraid I'd lose VIP keywords and GPS metadata when I opened that Library in Photos. Before moving to Photos, it was suggested I make a back-up of my 563 GB iPhoto Library on an external drive, then open original iPhoto Library on my Mac into Photos which convert it to Photos format. Then have the iPhoto app open the back-up on my external drive so I can compare the Photos and iPhotos Libraries to correct for missing info Photos. If that worked I would be safe to upgrade to new OS.

But back-up of iPhoto to volume on partitioned My Passport for Max external drive (1.5 TB of space) FAILED while I was asleep last night after running smoothly for about 6 hours. Also the external drive with 2 partitions volumes was inadvertently ejected TWICE, maybe what triggered iPhoto error message (not sure which was first). Wasn't aware of any power outage, not sure what went wrong.

Before backing up, I ran disk utility's first aid on external and DU said drive was fine. Also said my Fusion HD was fine. I have a recent Time Machine back-up, but this is only for converting iPhoto to Photos in advance of upgrading whole OS.

Suggestions besides trying a second time? And maybe making sure my Mac is not set to Sleep? I hate to lose painstakingly saved iPhoto data.