Thanks for calling me on this geneo, but I was questioning my own conclusions anyway and found I'd managed to bolux it up somehow
Redid a write test (drag and drop file copy). Folders containing NEF and PSD files. Copying from 4 TB Sabrient NVMe to either spanned or striped WD Red 6 TB SATA drives. Interesting results:
At 5 GB data (two folders): avg 1.5 GB/s for spanned; steady 1.9 GB/s striped (same folders)
At 25 GB data (5 folders): 1.9 GB/s decreasing to 150 MB/s spanned; 1.9 GB/s decreasing to 800 MB/s striped (definitely faster)
At 291 GB data (11 folders): didn't test the spanned drives; 1.9 GB/s decreased to a steady 325 MB/s before I stopped the transfer
My conclusion is that smaller transfers (5-10 GB) move really fast onto the striped drives as one might expect. For some reason, bigger transfers (longer time?) slow down to about double that for a spanned volume. But 25 Gigs go lickety split with the striped configuration! I could have timed these runs, but the "RAID 0" is definitely faster and I will go with that and do File History backups to an external USB 3.1. Should balance plenty of speed, capacity, and redundancy. As to why the rate of transfer drops, maybe it has something to do with buffers; curious.