I've done this with Raspian on the Pi before... Boot and swap partitions on the SD card and storage on a mounted USB hard drive to store all the movies and other media as it won't all fit on the SD card.
Anyone tried this on LibreELEC? looks like it might work, but I haven't tried it yet... save me some time if someone has tried this and could share their results/method
Yes the news PI's will boot from USB so they say, but it's marginal at best, and slow the few times I got it working.