It doesn’t really work that way. The hard drives that work in servers do not work in desktops and vice versa, for the most part. It is highly unlikely that your local Staples or Office Depot would have a hard drive that would actually function in the type of server that is usually used to provide Exchange Server services.
Moreover, I suspect that, forensically speaking, an investigator would easily be able to tell the difference between a brand new hard drive and one that had contained data and was wiped.
That was my thought. It would be very obvious to the FBI that this was a new drive. There would be at least some kind of time stamp on it and the complete absence of any data is evidence in itself.
It would be easy then, if the FBI wanted, to convict her of obstruction and destroying evidence. I doubt they would do it though.