Transfer rate between Hard Drives 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.
I would open up activity monitor and see what the effective instantaneous (and recent) transfer rate for data and more importantly iops.

Often many small files tax the total amount of IO operations and it's not a bandwidth issue at all. Large files that are stored sequentially on the filesystem (or when the platters and heads can not delay the transfer) will typically hit between 30 and 80 MB/s on a Mac Pro that's otherwise idle.

Similarly, more than 1,000 iops would saturate most pairs of SATA drives and perhaps far fewer IO could be limiting depending on catalog and file fragmentation or just long head seek times.