Hi there xobi555,
It sounds like you're unable to copy and paste from one drive to another because of this error number 8003. I would first boot to safe mode and test the issue there:
OS X El Capitan: Start up in safe mode
Starting up in safe mode may help you diagnose problems you’re having with your Mac.
Important: To print these instructions, click the Share button 
in the Help window, then choose Print.
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Choose Apple menu > Shut Down.
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After your Mac shuts down, wait 10 seconds, then press the power button.
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Immediately after you hear the startup tone, hold down the Shift key.
You should press the Shift key as soon as possible after you hear the startup tone, but not before.
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Release the Shift key when you see the gray Apple logo and progress indicator.
To leave safe mode, restart your Mac again, but don’t hold down any keys during startup.
If it works then restart as normal and test the issue again.
If not, then I would open your user folder in Finder, and remove a couple of preference (plist) files next:
1. Open the library folder - hold down the Option key, then choose Go > Library.
2. Open the Preferences folder
3. You're looking for com.apple.finder.plist, and you want to drag it to the desktop to create a copy.
4. Delete that file in the Preferences folder
5. Restart the computer and try again.
That should get you going again. Thank you for using Apple Support Communities, let us know how that goes!
