What is the test for reversal of a jury's verdict in a drug case?

In United States v. White, supra, 972 F.2d 590, the defendant was charged with simple possession of cocaine and conspiracy to possess cocaine. After deliberating, the jury indicated that it had reached a verdict on the possession charge, but not on the conspiracy charge. (Id. at p. 594.) After the verdict on the possession charge was collectively and individually affirmed in open court and recorded, the trial court directed the jury to continue deliberations on the conspiracy charge. (Ibid.) During the renewed deliberations, the trial court granted the jury's request to reconsider its verdict on the possession charge. (Ibid.) The jury then returned guilty verdicts on both charges. (Ibid.) The court in White concluded that the trial court erred in permitting the jury to reconsider its earlier verdict, reasoning that under the Federal Rules of Criminal Procedure the verdict became final once the verdict had been announced in court and unanimously affirmed during polling. (Id. at p. 595.)