Lexar's technical support was quite responsive and did their best to try to resolve the issue, but unfortunately were never able to find a solution. And, in all fairness, since I was never able to isolate a cause for any of this, there is an equally likely chance that the problem lies within the PowerMac itself. Ultimately, I held on to the JumpDrive to use as secure removable storage for the PowerBook, and purchased a different brand of flash drive to use when transferring files between the PowerBook and PowerMac.