I'll take a stab at this. Suppose the police go to a house with a search warrant to seize a stolen TV. They go into a bedroom and see the TV, but they also see drugs and sawed off shotguns. They will then amend the search warrant to seize the drugs and weapons. This simple step eliminates a LOT of headaches to justify seizing the items without a search warrant, because the judge who signs the search warrant has ordered the additional contraband to be seized. Then, if in court, a smart alec defense attorney challenges the officer and wants to know why he seized the additonal items, the officer tells him that the judge told him to. End of THAT line of questioning.
If a search warrant was granted to seize information regarding improper texting by Weiner and information regarding unauthorized storage of classified information was discovered, an amended search warrant could be sought to view, legally, the additional information which was found, so that criminal charges could properly be filed. If the FBI were to view the information without a search warrant and try to use it in court, it could all be thrown out, and then people would REALLY be pissed!
Now of course, what difference at this time, does it make? None, because Comey is still not going to file charges even if a search warrant is obtained. But if he at least goes through the proper legal steps, some future prosecutor can take up the case.
So the Huma/Weiner computer (well, actually its hard drive) becomes the critical item here.
Does the data it contains make it to DJT’s inauguration? Or does it go into the shredder?
In the second case, the case law is a little less clear. There is no definitive ruling by the Supreme Court and there is some conflict in the circuits. In most -- but not all -- circuits, the whole computer is regarded as one container and plain view again obtains. (You were looking for X under the warrant in the container, and could not help but notice Y.) But it would certainly be better to get a new warrant, just in case.
The DOJ guidelines are here: https://www.justice.gov/sites/default/files/criminal-ccips/legacy/2015/01/14/ssmanual2009.pdf Most of the perinent material in the instant case begins on page 19.