I would also be terribly interested in this.
I don't know how much blu ray discs are anymore, but if they are cheap, that's not a bad option, what is it 50GB per disc?
Dual Layer (DL) disks can hold 50 GB. But I'm a bit sceptical about Blu-Ray disks, because of the high data capacity. This means much more data per square inch, which requires a much higher precision both when manufacturing the disc as well as in the mechanics of the drives. With DL discs an even higher precision is required. I've just bought my first Blu-Ray drive though so I don't have much practical experience with them yet. One thing I experienced though is that the Blu-Ray drive was capable of reading some DVD discs without any problems which my DVD drives had problems reading (some sectors were reported unreadable).
The problem is the same with HDDs. A tiny error in the data layer on a low capacity disk may only affect part of a single bit, so that bit is still readable. On a high capacity disk (say 20 times higher capacity) the same error may affect several bits and make them unreadable. I've also noticed a lot more weak sectors (sectors that aren't so bad that they're unreadable and needs to be reallocated, but bad enough to take a long time to read) on high capacity disks. I bought two identical 2 TB disks last year myself, the one is OK but the other is extremely slow when it comes to read/write operations. Trying to figure out what was wrong I tried replacing cables, switch SATA ports etc., but nothing helped. Eventually I bought Hard Disk Sentinel and tested the drive, which revealed a lot of weak sectors. I haven't experienced that with disks with lover capacity.
I'm a hard drive guy though. I use file synchronization and back up the files in 2-3 separate drives. I'm also trying to build a bigger tower or rack to hold up to 20 drives and really automate the process even more. I think hard drives are the way to go for massive amounts of data, like terabytes you mentioned. One 4TB drive or 80 blu ray discs? In the early 2000s, I was backing stuff up regularly on cdr's, and i just don't have that kind of energy or desire anymore.
I prefer HDDs too, much easier to work with, and also safer in general IMO, at least if you back up on at least two separate drives. I also use online/cloud backup as an extra precaution.
I used to use DVDs, but then I always made 3 copies on different DVD brands, as some brands (or batches/types of the same brand) deteriorate faster than others. I recently transfered about 100 DVDs to HDD, they were several years old and practically all DVDs of a certain brand (Hyundai) were totally unreadably while the other brands I'd used were fine.
What is disturbing to me and complicates matters is that hard drives are becoming less in demand it seems since everyone has moved to mobile devices. So they are not as cheap as we are used to seeing anymore, nor are they increasing in size as quickly as they used to. Seems like 4TB is the limit currently and has been for a while. So I'm not sure what the future of hard drives are. I'd still prefer a hard drive over a SSD, but I'll wait until they catch up in capacity (probably a long time).
They're also coming close to the data layer capacity that's possible for HDDs. The more data per square inch, the more mechanical precision is required on all levels as well. That you actually can store 4 TB on a drive these days, and still maintain reliability, is impressive, IMO. It takes an incredible mechanical precision to be be able to do that, not to mention the speed the drives are operating at.
I'm curious if there are any side effects for hard drives that are sitting on a shelf for a long time (years). Should you plug them in once in a while to keep them happy or anything like that?
I've never had any problems with drives that have been stored for a long time (3 years or more). Just tried to plug a 13 year old Seagate drive which I haven't used for years into a docking station, no problems. Seagate has stated that the fluid bearings that has been the standard for about 10 years now could practically "run forever" if the rest of the mechanics didn't wear out, how they react to long time inactivity may be a different matter though. But 3 years or so don't seem to be a problem in general.
I sometimes have disturbing formatting issues that come up with these large drives that i plug in and out of computers. Let's say i put a bunch of data on a huge 4tb drive. Then i take it out. Later, I use it with an enclosure and a usb 3.0 connection. Then I do the same with an esata. Then i stick it into another computer. Sometimes, while doing this, Windows will say the drive is unreadable or something, or that it has to be formatted, or that it is corrupted. And I have a feeling that it is due to all the different cables/protocols the drive is being accessed with, but I can't really confirm it for sure. But when the message comes up and the drive is a critical step in your backing up process, it's pretty scary. I hate that feeling. I'm always keeping a close eye on developments with hard drives, esata/sata, usb 3.0, thunderbolt/lightpeak, enclosures, etc.
That problem can be caused by not dismounting (safe removal) the drive correctly when using an USB or eSATA connection (the write back cache is not written to the disk before it's shut down, as it should, which may cause data loss or corruption). Some eSATA drives like WD MyBook can not be dismounted when first mounted (Windows does not show the option), others can. When first plugged in you have to keep them running until you shut down the PC.