I use exclusively Western Digital MyBook single-drive USB3 externals - 6TB of internals + ~40TB externals with a passive external cooling fan (
tower exhaust). They've been going rock solid for many years, none have failed except one being dropped while accessing. I keep backups at my bro's place using all my smaller hand-me-down drives as I upgrade collections to 6&8 TB externals - he's got the few remaining Seagates, and they're kinda sketchy, he doesn't complain because they're free and I replace them soon after failure with another hand-me-down. Nearly all of my Seagate drives have died and were immediately replaced with Western Digital. The expandability is only limited by USB3 ports, and less important drives go through a powered usb3 hub; Surprisingly, the speed isn't hampered through a hub by much compared to drives individually connected to the motherboard... Backing up at USB3 speeds is not painful like USB2. Universal compatibility is also nice. I played movies off a 6TB drive through my phone's OTG on a road trip.
I know about NAS, but even with a gigabit connection, the transfer speed and portability of external USB3 is the most convenient for me. I heard you can use a cheap Asus router (RT-N66u ?) with a firmware flash to get two drives NAS'd for a lot cheaper than a direct hardware route, tho reading told me that WD's Mybook NAS solution would still be preferable. I have not looked into it much, so I can't recommend either solution.
I like direct memory access to my files through drive letters with Windows SMB Shares, PLEX and FTP for remote access through my server computer - that just works well for me. I can easily diagnose dropouts and other factors while the drives show up as drive letters like I'm used to. It's not so important anymore, but I regularly run
CrystalDiskInfo to see how my file integrity on each drive is faring through use. Mostly to decide which drive I pass on to my brother

.
Problems with Windows (Samba/SMB) shares?
Windows Share Manager is really slick.
PLEX is really cool. It's still got some problems - I've done the PLEX Dance⢠way too many times for me to still be using the software, but it has become essential for off-site viewing as I have to leave town often for work.
I know my friend has gone to NAS for a lot of his stuff, but it's still a hassle for him when something randomly isn't going perfect. It's one of those Linux things that the solution takes a lot of forum reading to fix...