Hi, tricosped -
Welcome to Apple's Discussions.
An "Invalid Extent Entry" error is one of several types of errors indicating damage has occured to the disk's directory. Although Disk First Aid can often diagnose that type of problem, it rarely can fix it.
Alsoft's
DiskWarrior utility can repair such problems. DiskWarrior comes on a bootable disk; since it does its work by replacing the disk's directory, the machine must be booted to a drive other than the one to be repaired.
If your iBook is OSX ready and can use OSX 10.3.9 or later, you can use the current version of DiskWarrior. v. 4.1.
If it is not OSX ready, you will need to use an earlier version of DiskWarrior, v. 2.1 or 2.1.1. You may be able to find one of these at places like ebay (doubtful) or borrow one. I understand you can obtain one directly from Alsoft via telephone order.