"Secondary" backup? So a backup for your primary backup?
That and a backup for data that I don't regard as being that important. Also for burning ISOs or being able to drop off data with customers in scenarios where waiting for a transfer to complete isn't convenient.
We're pretty much there already. A 100 spindle of DVD-Rs cost about $20. That's 4.38 GiB each, or 438 GiB total and $0.0457 per GB. A 3TB drive nets about 2770 GiB. At $100 each that's $0.0361 per GB.
Considering that I still need to buy DVD recordables for the other usage scenarios I described, it's not a case or 'either or'. Furthermore, if I was going to buy a new HDD for backups, it would be a portable drive for size and ease of storage which comes in at £65 for 2TB, versus £20 (100 DVD spindle). My income makes a purchasing decision involving £20 a lot easier than one of £65 or more, especially when talking about backups for (most of the time) less important data.
And even cheaper when you think that your entire 100 disc spindle can be replaced by a single retired 500GB disk drive.
That would be when a retired working HDD comes along, which isn't that common in my line of work.
I don't see having two hard drive backups as "vulnerable in the same way". It's not like both will be rendered useless if one is lost in a fire or flood while the other is in your safe deposit box on the other side of town.
While it's useful to look at possible/specific disaster scenarios, considering that disasters (I'm using that term in a personal and relatively speaking way) don't often come in predictable forms, I think it's useful to simply look at the storage medium and analyse its vulnerabilities. I look at HDDs and see the following:
1 - Vulnerable to physical shock
2 - Vulnerable to strong magnetic fields (admittedly I have no idea how strong a field is required to cause problems)
3 - Easy to read/write (say if I made a mistake and my system got infected in some way, and I connected the backup device)
4 - Vulnerable to water damage
5 - Vulnerable to sudden drive death
6 - Vulnerable to steady drive death (bad sectors)
Of those, optical media is vulnerable to points 4 and 6. Optical media also has its own vulnerability that sometimes optical drives don't like certain formats or brands of disc (which is one reason why I have two optical drives connected to my PC). Optical media is considered not to have great longevity partly because of its increased vulnerability to point 4, but I haven't personally experienced this and I have recorded discs dating back about 15 years (which I've tested from time to time out of curiosity).
Considering a single layer DVD's lack of capacity, it's not as likely to get damaged in the same way that a HDD could be; I have little reason to get my backup DVDs out often whereas a backup HDD is likely to be in use as I would be backup up to it on a regular basis.
On the other hand, because I'm not likely to be reading or writing to a particular disc on a regular basis, one could say that undetected creeping death for optical media is more likely unless the discs are tested reasonably regularly. But then any backup device ought to have its data integrity tested.
It's easier to catalog 100 discs than it is to catalog a single disc?
Admittedly my point here had flaws. When I back up to a DVD I immediately note in a spreadsheet what I've backed up and when, and I also number the discs. Retiring a DVD is less problematic, I simply snap the disc in two after checking that the original/live data is OK, and there's less data to consider. I'm always more nervous about deleting retired files from say a backup HDD.
Sorry. One HDD is much easier to store than 100 DVDs. Smaller, too.
The internal 3.5" HDDs I have aren't easier to store (IIRC, 1x 250GB, 1x 320GB, 1x 400GB and 1x 500GB). While I have two nice Samsung plastic cases that came with some drives I once bought, they're not common in my experience, which means I have to otherwise bag the drives in anti-static bags and stack them, double-checking that when I stack them, the circuit boards aren't flat against the surface of the drive beneath. This normally isn't an issue if the drives are identical brand ones, but mine aren't all the same brand. Storing them in a stacked criss-cross fashion would probably be safe, but bagged cross-cross stacked drives use up a lot of room and it's not easy to determine which drive is which (unless I start putting stickers on the bags).