Assuming you're talking about a standard magnetic "hard drive", yes.
With the right equipment, the magnetized areas (bits) can be written, or re-written, in any order desired. It's possible that it could be done without connecting the drive to other equipment.
Don't misunderstand though. Neither of those actions would be reasonably expected to have been done. Admitting a "possibility" is easy. It "could" have been done. But the likelihood of it having been done is so slim that it should be ignored unless significant evidence of it is shown.
If the normal operating system shows no obvious evidence, finding it is likely to take the same level of expertise, or higher, as was needed to make the alterations.