Is the destination drive used for Time Machine backups? Then you cannot move a Photos Library there because of permission problems.
You did not yet tell us, if the "Ignore Ownership on this volume" flag is available for the destination USB drive as shown in the screenshot posted by OT? Did you enable it?
Copying between drives with different filesystem formats may result in copy errors, because the supported filenames may differ, for example, if if one drive is case-sensitive and the other not.
I hope, the solution to copy the library with PowerPhotos will work.
If not, you try to identify the file, that is causing the read/write error. You can do that, by copying the subfolders of the library separately.
This way you can see how far the copy got, before the error occurred. The first file that did not copy will be the culprit.
- To open the library package select the library in the Finder and Ctrl--click it.
- Select "show package contents".
- Then create a new, empty folder on your external drive and drag the folders from the library one-by-one into the new folder.
- If a copy error occurs, check the destination drive, how far the copy succeeded. The next file to be copied, is probably causing the error. Skip it.
- Once the library has been copied, rename the folder with the copy to "Photos Library.photoslibrary". Now the icon of the copy should change from the folder icon to the Photos Library icon.
- Try to open the copy in Photos by double clicking it.