he used an electronic doo-hickey that beeped for each one

All that means is that the connection was good when he tested it.

If there is a bad solder joint on the IDE header, the very act of testing it (pressing the probe against the header pin) could well have completed the connection, causing it to test good.

Mark is right -- you need to re-melt each and every one of those IDE header connections to be sure that they are reliable.

tanstaafl.