it appears that copying files to a USB drive from the MAC OSX El-Capitain hands when using Finder.
It first happened between two USB drives both inserted in the iCAN Double SATA Hard Drive Dock/Stand-alone Duplicator.
Instead I copied music files from the old USB SATA drive to my MAC's hard drive, with the idea to move them from there to a new external SATA drive. I found that if there were many files in the copy request, the copy process would hang. I'd have to try to relaunch Finder (alt-cmd ESC) but this would not work either until I powered off the USB dock. When I reboot the MAC and restart the USB dock, the MAC complains that the USB was not ejected first. (not able to)
copy failures that I am experiencing.... and others too...
http://www.mac-forums.com/os-x-operating-system/273325-usb-3-0-external-hds-freezing-copying-files-finder-freezes.html
I am hoping to get around this issue by copying from the console.
Donalds-iMac:~ MAC_at_home$ mount
/dev/disk1 on / (hfs, local, journaled)
devfs on /dev (devfs, local, nobrowse)
map -hosts on /net (autofs, nosuid, automounted, nobrowse)
map auto_home on /home (autofs, automounted, nobrowse)
/dev/disk3s2 on /Volumes/macam (hfs, local, nodev, nosuid, read-only, noowners, quarantine, mounted by MAC_at_home)
/dev/disk2s2 on /Volumes/disk2016 (hfs, local, nodev, nosuid, journaled, noowners)
Donalds-iMac:~ MAC_at_home$ cd /Volumes/disk2016
Donalds-iMac:disk2016 MAC_at_home$ ls
Books Music archive
Donalds-iMac:disk2016 MAC_at_home$
Donalds-iMac:disk2016 MAC_at_home$ ls
Books Music archive
Donalds-iMac:disk2016 MAC_at_home$
------
from my temp music directory I tried:
MAC_at_home$ rsync -av . /Volumes/disk2016
but it hangs too. :-(
I looked at the error log, and found lots of airserver errors, even though I was not using it, so I removed it.
I will have to try copying music on linux