LorenAmelang@dmerej Thanks for confirming my lack of confidence in filesystems not native to Linux.
Actually my new drive is exFAT. I dug a bit and found Arch sees all permissions granted on every file on the drive. Also, stat -f shows the actual cluster Block size: 131072, which wastes a ton of space for things like tiny Node modules! (All the other disk info commands show 4096, which is definitely wrong!)
So I've set up a separate little EXT4 SSD for the 'systemy' parts of my backup.