So, the second Powerbook has had its fuse repaired, and I touched up the soldering on the first PB as well, thanks to new flux and the awesome video at CuriousInventor.com

However. One Powerbook runs just fine with its 1.3GB HD. I put a Toshiba 6GB drive in the second Powerbook (which is what it came with), and the drive would spin up and immediately spin down again. I know the Toshiba works, as it spins up fine on my USB adapter board. However, the Powerbook also works, because it boots off the 1.3GB drive from the other Powerbook.

It looks as if there may be some truth to the notion that newer drives aren't good for the 2400c.