I am on a 2012 Mac Pro. I just asked it to copy my iTunes music folder which happens to be 40GB. Now, SATA shows a 3Gb speed, so ~320Gb of data should take all of 110 seconds. I know 3Gb is the high end for the SATA itself, and the drives are much lower. So instead of 2 minutes, even 20 makes sense.
I'm looking at 26 hours? Any ideas what can cause this? Fortunately, the need to back up this amount of data is rare. Just wondering what's up.