What would be the down side(s) to backup archival via SD cards?

Solution 1:

I'd say that there are two drawbacks: First, you have no way of knowing what file is archived on which card, so search/retrieval becomes an issue. But the main drawback would be the cost: an external HDD is going to be orders of magnitude cheaper than SD cards.

Solution 2:

NAND flash doesn't hold its charge forever. The electrons trapped in the floating gate slowly leak out. How long it remain readable depends on the density of the flash. The 3xnm flash found in most SD cards today has a retention time of around 5 years. The 2xnm flash that is starting to show up now has a retention time a bit lower than that. How much lower I don't not know exactly. 4 years or something like that I supect.

Solution 3:

Not to beat a dead horse, but two other downsides that jump to mind:

-not being able to easily determine if the data was "changing" on the cards (e.g. if you've got things relatively centralized on a hard drive, you could easily do something like occasional MD5 checksum checks)

-much more difficult to do regular offsite backups (a fire or theft could easily cause a problem long before bit rot)