copying stopped at 98.39%, as the disk I was copying to ran out of space. I had accounted for this at the start, as I figured the dying disk has 700GB unallocated space at the end of the drive so it's probably fine. total estimate for the run was 4 days and 21 hours, and we ended it prematurely at 98.39% with a run time of 4 days, 12 hours and 3 minutes. estimate was pretty good tbh.
time for sleep now though, I'll inspect the results tomorrow