My goal is to create an simple offsite backup of my CasaOS setup using a RasPi 3b+ with external USB drive at a friend's house. Are there any recommended methods for doing this?
Also: what should I look for in an external hard drive as far as reliability goes for something that will essentially always be on? I'm not well versed in all the WD blue, red, etc. Does it matter?
+1, all of my SBCs that do important things have the OS installed on an SSD plugged into a USB3 enclosure and (if needed) only the bootloader and /boot on an SD card. That cuts wear on the SD down to almost nothing. Pick up a good condition msata or m.2 2230/2242 drive on eBay and stick it in an enclosure and watch SMART stats and you shouldn't have any unexpected failures.