Martha Helena writes in with a problem that may have a few different angles. Her startup drive started to fill. Her Photos Library file is 12.5GB, and she had just a few gigabytes left on the drive. While she was able to copy an older iPhoto Library to the other drive, the Finder kept hanging up during the Photos Library transfer with this error:
The Finder can’t complete the operation because some data in “Photos Library” can’t be read or written. (Error code -36)
A decent number of people have received this error with an iPhoto or Photos Library, possibly because the media library folders are big folders full of other folders. This increases the odds that if you have a faulty file, it will scotch the copy. Because the library files are packages which look like files, the error gets reported for the package name, rather than revealing which file within the package is at fault.