"Once those items are in the possession of the expert, the expert plans to make a "forensic copy" and then check to see if anything was accessed or altered."

If the seized computers were booted up from their normal OS drive then a whole bunch of timestamps, metadata and logs would be changed. A forensic image of the drive could be used to prove this without altering the original device.

Of course if the police used a forensic image tool in the first place that would leave far less evidence of analysis on the original machine.