Copies are fine, provided there is reliable chain of custody, method of making copy and then sequestering of copy.
You have a good point about getting the original at some point though, e.g., getting Comey's actual computer. Sometimes network and backup systems provide reliable record of what was produced.
HO, Ho, Ho. I think Comey is without his computer, and it should be in the safe hands of the FBI, by now under Director Wray. (Tongue in cheek.)
Remember Trump fired him while Comey was in California. They biked over his termination papers, and sealed his office, is that right?