One by one would be great, but when there's 40,000 files ya got no choice but to copy them in batches.
I'm gonna have to look into some kind of automated backup system that double checks things automatically or something. It literally took since last Friday just to back up files and install one drive. I figured all I'd have to do is switch it out, copy all the files and presto. Suprised the hell out of me when half the files didn't finish copying.