I encountered this exact same problem recently (the corruption looks just like the frames posted). Using a PC laptop with XP, transferring from the PCMCIA slot to external hard drive using USB. Footage on cards was fine, but footage on drive included these artifacts randomly. The artifacts are difficult to detect using P2 viewer (which does not play back every frame on my system).
I have used the same procedure for transferring P2 footage for three years. I have transferred hundreds of P2 cards with no problem, including a feature film with dozens of hours of footage. No lost clips, no dropped frames. Now this.
I switched out external hard drives. I switched out cables. I switched out laptops. Still no dice. Blaming an environmental factor like an EM field could make sense, but it would feel like passing the buck.
The only workaround I can think of would be to copy each card twice, so that randomly induced errors would be in different spots. But there simply isn’t time for that in most shooting environments. And it shouldn’t be necessary.
I’d love to hear if anyone else has this problem, and if they found a solution.