- Synology as at-home Time Machine (TM) backup. Synology also has its own single drive failure, RAID 5-like backup option active. So a drive could randomly conk, I get an alert, replace the dead drive, let Synology rebuild to incorporate the drive and I've lost no data.
- Big HDD as at-bank (safe deposit) box backup. This disk is partitioned: TM backup up into a few TBs partition 1 and, in the rest of the space, I use Chronosync to sync specific files & folders like Documents, Music, Photos, Work folders, etc. Yes, this means that I have TWO copies of select personal files on the disc at the same time but that's OK by me. Some of the stuff I back up into this space is excluded from the TM target (for backup). Chronosync is ideal for this selective files & folder backup IMO.
- Second Big HDD at home set up and used just like #2. #3 & #2 rotate between home and bank safe deposit box roughly every 4 weeks (and just before any travel where the risk of theft of #1 & #2 + Mac is highest).
- Key work folders of original content regularly synched Desktop to/from Laptop, effectively yielding nearly 2 live copies of key files too.
- Key personal files like home movie collection shared with other family members in other states, updated with new ones when I visit them. These are the most distant backups of original files I would hate to completely lose.
To completely lose what is backed up in #1-4, a chunk of my state would have to be taken out at the same time, with both Desktop & Laptop in the devastation zone. If so, apparently it's a no-notice event (asteroid, nuke, etc) which likely means I'm there too, so I perhaps won't be able to care about data loss in that scenario. Else, if there's notice (major Hurricane for one), I can load up the laptop and take desktop + one of the big HDDs with me to the evacuation zone. Home & Bank could be completely destroyed and I would still be able to recover everything.
There's probably a flaw here but it is not obvious to me. Through my lens, it's probably overkill to have both Synology + #2 & #3 as- effectively- 3 complete TM backups. I'm going to the bank roughly every 4 weeks anyway, so it's no big hassle to swap the HDDs while I'm there.
And yes, a cloud option could be added to the mix (perhaps replacing #2 & #3 with no trips to the bank) but I have no interest in cloud solutions. IMO- injecting strangers between my data and me seems to fundamentally miss the point of completely secure backups. I'd rather have my own 100% controlled "clouds" in the form of HDDs that float back and forth from home/office & bank safe deposit box.