Okay, FULL EXPLANATION:
I erased the Hard Disk while it was installed in my T2, via USB using my Mac. I used the "Disk Utilities" and selected the option that erases the HD and writes everything over with zeros so that data cannot be recovered.
Now, this erased HD works nicely on my Mac, and even the T2 recognizes it is there because it comes up as "HD1" in the format window, but every time I try to access it on the T2, it says "Disk Error!"
Yes, I reformatted it to DOS FAT 32, which means it should be in original new state, but it still does not work on the T2, and formatting it using the T2 doesn't work either.