(Copied from superuser)
With SSDs getting cheaper and more mature, I am thinking of getting some for off-site backups. I would store system images and media backups on it, updated weekly-monthly. The drives would be rarely moved (although some shock tolerance would be welcome), stored at ~20C and powered on only to update the backups (so weekly/monthly). Considering all this, would I be better off with an external SSD or HDD?
(SSDs - consumer grade MLC/TLC, Samsung T5, HDDs - consumer/enterprise grade (not much of a diff anyway) WD, both in under 1TB varieties)
Assuming price is not a concern, neither one has much of an advantage over the other for your purposes.
A powered-down laptop hard drive is reasonably shock-resistant: the 500g rating is roughly equivalent to falling off a table onto a concrete floor.
A SSD has a much higher random-access speed than a hard drive, but backup and restore are mostly sequential-access operations, where SSDs don't have anywhere near as much of an edge.
Hard drives have better long-term powered-off behavior. A hard drive pulled out of storage will either power up properly, providing access to all its data, or not power up properly, requiring work by a data-recovery company to provide access to all the data. A powered-off SSD will gradually lose data in an unrecoverable fashion after a few years. This is mostly not an issue for you, since you plan to store the drives at low temperature (which reduces the data loss rate), and you plan to power them up periodically to update the contents.
SSDs handle being turned off and on much better than hard drives do, but again, this is mostly not an issue: being power-cycled 50 times a year is well below the limits for a hard drive.
The big advantage hard drives have is still price: a 1TB external hard drive is about $60, while a similar-sized SSD is around $300.
Whichever way you choose to go, make sure your backup plan can handle the failure of one of your external drives. There's no point in having a backup if the backup itself is a weak point.