1) my drives stay mounted all the time and my computer stays powered up 24x7. Why? Because many HW failures occur on power up-shut down-power up cycles. If hard drive wear from spinning is a concern (it shouldn’t be, the platter spindles “ride” on air bearings), there is SW to spin down drives when not In use, but with SSD prices dropping every year, 2TB SSDs are not that much more expensive than an enterprise drive of the same size and have MANY advantages. Whatever you choose to do, you still need that off-site (cloud) DR copy of your data to protect against exactly the failure you just experienced. The fact that you couldn’t find all your data on Backblaze means it wasn’t uploaded or organized properly or you need to choose cloud storage from a MAJOR provider such as Amazon, Microsoft, Google or Apple. With cloud storage, like everything else, you get what you pay for.

2) What do you see as the advantage of a NAS? The only real advantage of NAS vs direct USB connected is that with appropriate security precautions, you can access your home data from a remote location. BUT it’s slower and since the NAS box has the file system, which is usually proprietary, it won’t be as robust/as well tested/as well supported as NTFS. Remember, it’s the file system that handles your data, so a robust file system is super important and consumer class NAS file system designers just don’t have the resources as a Microsoft. You can share data from the NAS with other devices on your home network, but you can do that by simply mapping drives.

3) A RAID 5 system does provide protection from a single drive failure. It doesn’t mirror per se, but parity information, which can be used to reconstruct missing data on the fly, is rotated across all the drives. It requires 3 drives minimum and you should have a spare that the system can mount and rebuild in the event of a failure. During that rebuild period (which can take days with big drives), the speed drops in half and a second drive failure will cost you all your data. On some systems you can add an additional drive (a so called RAID 6) to protect against a double drive failure.

A RAID 10 (1+0) is faster than a RAID 5 but “wastes” more space for protection - you get 50% of the total storage as usable, while a 3 drive RAID 5 gives you approximately 66%, or the same usable space as a 4 drive RAID 1+0. Depending on where the two drives are that fail in a RAID 1+0, it MAY tolerate a double drive failure without data loss. Personally, I would choose RAID 1+0 over RAID 5 - it’s long been the choice of classical data base administrators for a reason.

Note that none of these choices obviates the need for a backup (a RAID is NOT a substitute) AND an off-site disaster recovery copy with versioning.

Hope that helps. I commend you for taking steps to prevent another loss of data and understanding the choices open to you.