When the government seizes computers or other large data digital storage media, it usually removes the items from the search site to a forensics lab because, it argues, a true forensic search cannot be done in that location or in that time frame. A year or more later, perhaps there is an indictment.
Most of the time, though, that seizure and removal encompasses more than the target of the warrant. As we know from today's decision in Riley/Wurie, technology can and does store vast amounts of personal information. How long can the government keep information seized pursuant to a lawful search but that, as it turns out, is not responsive to the warrant? How long can it keep family photos, music files, and other everyday, innocent, and usually boring detritus?
In US v. Ganias, the agents didn't actually take the computers, but made mirror image copies of three hard drives, which contained much information outside the warrant. In other words, it was not what they were after. At least not at that time.
Another later investigation led to questions about those documents, and two-and-a-half years later, the government got a warrant for the information already in its possession. Those documents no longer existed on the defendant's computers. The issue and the answer, in short order,
[W]hether the Fourth Amendment permits officials executing a warrant for the seizure of particular data on a computer to seize and indefinitely retain every file on that computer for use in future criminal investigations. We hold that it does not.Then, the Court found that information should be excluded because "the agents could not have had a good-faith basis to believe the law permitted them to keep the non-responsive files indefinitely," rejecting arguments that the defense should have asked for it back and that it was too much trouble to delete the non-responsive information.
And, finally, deterrence, the heart of the exclusionary rule.
"[T]he benefits of deterrence in this case are great. With the Government's use of forensic mirror images becoming increasingly common, deterring its unconstitutional handling of non-responsive data has grown in importance." So while temporary retention may be reasonable, indefinite storage of intermingled information outside the scope of the warrant is not.
From the Volokh Conspiracy, Orin Kerr's take on Ganias.