iPhoto gives an error message when trying to open a library

If you have a library that, when you attempt to open it in iPhoto, you get an error message saying "The iPhoto Library is locked, on a locked disk, or you do not have permission to make changes to it." (or something similar), there are a few basic reasons for the vast majority of such errors.

Two copies of iPhoto are trying to access the same library.

If the library you are trying to open is being accessed over a network, such as through file sharing from another machine, or a network disk such as a Time Capsule, Airport Extreme, or other NAS drive, make sure that no other machine on your network already has that library open in iPhoto. iPhoto only allows a library to be open in one copy of iPhoto at a time, so if you already have the library open in iPhoto on one machine, you will need to quit iPhoto on that machine before you can open that same library on a different machine.

You do not have sufficient permissions to open the library.

iPhoto requires that you be able to both read and write to a library in order to be able to open it. This is the case even if you don't plan on making any changes to the library - you still need to have write permission to open it, read-only permission is not enough. Here are a few troubleshooting tips to make sure you have write permission to your iPhoto library:

• If the library is on read-only media such as a burned CD or DVD, or even a USB stick that has been put in read-only mode, you will not be able to access the iPhoto library directly from that disk. To open the library, you will first need to copy it to a hard drive where it can be modified, then add that copy of the library to iPhoto Library Manager using the "Add Library" button. You can then open and view that copy of the library.
• If you are accessing this library through file sharing to another machine or on a network disk such as a Time Capsule, Airport Extreme, or NAS drive, make sure that you connect to the network disk as a user that has write permission to that disk. For example, if you connect to another machine as a "guest" user, you will almost certainly only have read permissions to the iPhoto library. You should instead connect using the username/password of the user that owns the library on that machine, or as an admin user.
• If your library is just on a regular internal or external hard drive, then trying using iPhoto's built in permission repair function. You can read about using that on Apple's website at Rebuilding the iPhoto Library.