The cancel dialogue will let you know when the scanning is expected to finish. You can cancel a scan, and then have the scan resume (from where if was cancelled).
A 2TB USB disk takes about 10 hours, and needs about 75 M bytes of storage space depending on file sizes, and whether the virus scanner is running. Since the files are already on the disk, there is no harm in temporarily switching the on-access virus scan off!
When the scan is finished the cancel dialogue vanishes and the “normal” window reappears.
Now is a good time to save the database to a disk file.