Both drives are formatted for mac files.
Is one drives Mac OS Extended Journaled Case Sensitive?
Lets see if we can trick the drive into letting us copy the library over.
_Go to Go > Utilities > Disk Utility
_Once in Disk Utility go to File > New Disk Image from Folder
_Locate and click once on the iPhoto Library on your external drive and then click the "Image" button
_Save the disk image to the desktop and set "Image Format" to "read only" and "Encryption" to "none"
Assuming the disk image is created successfully you can then double-click on the image on the desktop and it will mount and open to reveal your library. The library can then be dragged in the ~/Pictures/ directory.
Note that you will have 2 copies of your library on the computer at this point. 1 inside the image, and one that you dragged to ~/Pictures/. Once the copy is complete and iPhoto is working you can get rid of the disk image.
If you encounter the same issue with the Disk Image creation that you did just copying the iPhoto Library over, it might be a good idea to reformat this external drive and put a fresh copy of the iPhoto Library on it from the original source.