Howdy and welcome,
OK, so the drive you added but that the Mac could not see did work OK outside the PB, yes?
Two things to check regarding your replacement HD:
1. Older Macs need the boot drive jumpered to Master.
2. Apple's tools for initializing (formatting) a drive is not as robust as what you are used to with Win, so it is best to erase--unformat--the drive in a PC, if it is currently something else like NTFS.
Besides that, need to know which model Mac (
this one?), and which OS you are working with. Details help.
Sounds like a project......