How can I resolve Error code -36?

I'm using a 13-inch, Mid 2012 2.5 GHz Intel Core i5 Macbook Pro, and it's running Yosemite 10.10.5. I use a 1TB external hard drive from WD, which houses my Photos Library (about 200 GB).

I have begun to work remotely, so I have tried to switch to using a Toshiba 2TB portable hard drive so that I can access my Photos Library and other files remotely, with the intent of using the WD external solely for overnight Time Machine backups. The problem began when I attempted to move my Photos library to the portable drive. About 3/4 of the way through the 3 hour transfer, I got the error message, "The Finder can’t complete the operation because some data in “Merged Library2” can’t be read or written. (Error code -36)".

I've gotten the same message when attempting to simply move the Photos Library to my desktop as well. Other files move seamlessly, but it seems as though my Photos Library is being held hostage by my external drive. One issue to note is that my Photos application has been stuck "Uploading 12 items" for days. With the exception of these 12 items (whatever they are), my entire library is on iCloud.