FWIW: I store everything...on hard drives. I have an excess number of hard drives and I see no reason not to put them to use.
Considering the cost of drives today, that seems as safe as snything, IMO.
If I wanted to try to be extra safe, I would store them on drives which I don't subject to regular use but I would not use an external drive since such introduce one more thing (the enclosure) which could fail/cause problems. Internal drives can easily be removed from the system and placed in a container...until access is desired.
I don't think SD cards are intended to be anything but short-term storage...due to the volatility of the media.
See similar discussion at http://www.dslreports.com/forum/r19794656-CDRs-or-Flash-Memory-for-long-term-storage