SSD or HDD for data hoarding?

Hot swap HDD's in a NAS with hardware raid and hot spares. Search for "hot swap drive bay NAS".

If it doesn't need to be portable ("shock-resistant") then the most cost-effective medium is HDD.

But don't forget that you must be able to back it up on at least a daily basis.

It's not really hoarding if you can afford to ask this question.

used HDD