Can't copy iPhoto Library- "the alias iPhoto library cannot be copied..."

I am attempting to backup my iPhoto library contents on my iMac's internal drive (OSX 10.5.8) by dragging my iPhoto Library ( iPhoto 9, 8.1.2) from the "Pictures" folder in Finder to a firewire connected Ext HDD. The external drive is formatted Mac OS Extended (journaled) and has plenty of available space. I have tried several times but, briefly after the file transfer starts, I get the message " The alias "iPhoto library" cannot be copied to the destination, perhaps because the destination does not support this type of alias". From what I've read here, my library is set up as a "managed" library- imports are copied to the iPhoto library. To my knowledge, I have no photos located elsewhere that are referenced in the library. I've had no problem copying other files, including my iTunes library. Any ideas? Thanks much for your help!