is there anything of value on the drive? if so try to back it up using the other computer
anything you do, has the chance to totally distroy the existing data,...... you have been warned
if you had created a partition, i would have recomended removing/recreating the partition using the exact same parameters, which would have refreshed the boot code
i suspect that due to no partition being set, that the os was able to write data where it should not have been allowed to
IE- the os thought it was writing near the outside of the data plater, but actually wrapped around towards the begining of the plater data and overwrote existing data
my best advice is to erase the drive, and this time create a partition
better yet, get a 2TB drive for C:\ and use the 3TB as a data drive in a usb enclosure where it 's full capacity can be seen/used as the usb driver will allow the drives full size to be available