IPhoto stores the location of its current library as a simple path, so if you open up iPhoto with a library on your external drive set as the current library it will go looking in /Volumes/External instead of /Volumes/External-1. 'External', 'External-1', 'External-2', etc. In fact, if the same crash/outage/unplugging happens multiple times, you can end up with multiple folders, e.g. This is not immediately obvious though, since the name of the drive appears the same in the Finder. So, what it does instead is to mount the drive at /Volumes/External-1 (or sometimes '/Volumes/External 1', depending on your version of OS X), so the drive's contents show up there instead. The next time you plug in the hard drive, OS X will want to mount the drive at that same /Volumes/External location, only now there is a folder there, so it can no longer use that precise location. This folder is actually stored on your internal hard drive, and any program that writes files into it will actually be writing those files to your internal hard drive when it thinks it’s writing them to the external drive. Since there is nothing at /Volumes/External, a plain folder can sometimes be created at that location where the drive contents used to be. Many programs will not handle this well, and will continue to write files and folders to /Volumes/External even though the hard drive is no longer there. The most common trigger for the disappearing library problem is when you either have a system crash, a power outage, or you accidentally disconnect the external drive without ejecting (i.e.

(for the rest of this explanation, substitute the name of your own hard drive for 'External') So for example, if your hard drive is named 'External', OS X will mount your hard drive at /Volumes/External, and all the hard drive's contents will appear as subfolders underneath that. There is actually an invisible folder named 'Volumes' at the root level of your main hard drive, and when you attach an external hard drive to the machine, OS X puts the contents of that hard drive inside the Volumes folder. makes it ready for use) in a particular location on the system. When you attach an external hard drive, OS X 'mounts' the drive (i.e. you open up the library iPhoto and it contains no photos or albums), there is one particular cause that accounts for a large number of these cases. If you have an iPhoto library that you are keeping on an external hard drive, and that library suddenly has all its content 'disappear' (i.e.

How to clean up iphoto library