I am unfamiliar with the IOmega external 1TB hdd, as I only own ZIP and JAZ drives from IOmega. If it is 1TB in size as you wrote, I would guess that it is not terribly old, like my ZIP and JAZ drives are.

Since you have it connected to your MacMini, I am guessing that it is either a USB or Firewire externally connected drive, and since you mentioned that you also tried to use it on your Windows7, 8, & 10 machines, I'll guess it is a USB drive, as most people don't have Firewire ports on their Windows PC's.

Glad it is working for you. The Poseidon USB stack included within MorphOS is very good at connecting to most USB storage devices, as well as some other types of USB devices as well. I hope that more work is progressing to update Poseidon (or another USB stack), to soon give us the capability to use USB sound devices, video capture devices, etc.