I would go for SSDs if I needed speed. SSDs longevity is just fine. Any drive can die when you leave it unused for a decade.

Well, I'd say that a single device can't be both easily accessible and secure. A NAS like Synology wold be my choice for simple and convenient access and then another backup to B2 with rclone.

Both. Or HDD/SSD plus cloud like Backblaze Personal or Wasabi. The main thing here is to have several backup copies. Any drive can fail unexpectedly.

I understand it doesn't have to be a NAS but it would be much better in my opinion. RAID enclosure over USB is...well, not the most reliable solution. I would look into some Synology options. As has quite nice DSM and it can combine drives of different size in the SHR RAID: https://kb.synology.com/en-af/DSM/tutorial/What\_is\_Synology\_Hybrid\_RAID\_SHR

I honestly don't see a huge need for a NAS in your case. Especially, if you still have free space on the HDDs. I mean, a single HDD can cover your needs easily and I don't see any benefits in a NAS for backups.

I don't think it matters that much. It's more important to have several backup copies and follow the 3-2-1 rule: https://www.backblaze.com/blog/the-3-2-1-backup-strategy/. For backups, I would just go with a cheaper option.

I would add that flash drives are also less reliable. If you store some important data there, I would go for an SSD.

ZFS can do self-healing if you drives are in mirror or RAIDZ1/2/3. I would put those two drives in ZFS mirror. Then you would scale it by adding another two drives in mirror.

ZFS doen't need much RAM. All that talk about 32GB RAM or 1GB per TB storage is nonsense. It will work with any amount of RAM. Primarily, ZFS needs more RAM if you're using deduplication.

Keep several backup copies. HDD plus SSD and keep one drive in some remote place. Or SSD plus cloud like Backblaze B2. Overall, follow the 3-2-1 backup rule: https://www.backblaze.com/blog/the-3-2-1-backup-strategy/

SD cards are not the most reliable media in general. I would just go with an SSD at this point.

Well, I would say, keep multiple backup copies. An external drive plus cloud like Backblaze B2 which I think is exactly your case in terms of price. Upload encrypted with rclone.

Well, first of all, raid is not a backup, it's about uptime. So keep separate backups if you need your data safe. As to raid, ZFS or MDRAID but i would go with ZFS raidz2 or RAID6 on that capacity.

Cheaper than backblaze Personal? There is B2 or Wasabi but it also depends on the amount of data you have. Regarding the issue if you don't have an internet connection, Backblaze Personal has limited version history but you can expand it: https://www.backblaze.com/cloud-backup/features/extended-version-history

I would just avoid SMR drives, especially if you're looking to use them in RAID: https://nascompares.com/answer/list-of-wd-cmr-and-smr-hard-drives-hdd/ . Otherwise, any drive that fits your budget and that is CMR.

There shouldn't be any problems with that. Here's basically your question: https://www.truenas.com/community/threads/transfer-data-from-usb-hdd-to-freenas-box.24226/

Agree with the NAS suggestion. Will be more convenient and reliable than a USB RAID enclosure. I would look into Synology if you want a ready solution.

Because of quite high chance of getting an SSD that was overwritten multiple times or an HDD that was used a lot which increases chances of it's soon failure. But again, used drives might not be bad if you're buying from a reputable seller and depending on what you use them for.

For automation, I would look into some proper backup software like Veeam which has free Agent for Windows: https://www.veeam.com/agent-for-windows-community-edition.html. Also, try following the 3-2-1 backup rule: https://www.backblaze.com/blog/the-3-2-1-backup-strategy/. For example, you could add Backblaze Personal cloud.

Well, I was ranting about internet speed being lower than advertised after optics installation which should be 1GbE (I got only 300MbE) until I understood that the Asus router I bought a couple of days before that was just 300MbE.

If you have money for an SSD system, then why not? Also, there is no "no need to modify me for an eternity system" as such. Backups and periodic verification is a must if you want to keep your data. No matter HDDs or SSDs. Also, RAID 1 with 4 SSDs? That's only 1 SSD usable capacity. Unless you mean RAIDZ1.