"i have 3 pc's
(win 10, win 7) none of which will "see" the biab HD whenever i plug it in."
You need an hfs+ driver for windows https://forums.macrumors.com/threads/apple-hfs-windows-driver-download.1368010/

Click on the main User iMac icon under Devices in Finder and see if it's showing in there as it may not be set to show in Finder Preferences Sidebar Devices External Disks.