They’re still mounted individually, so you do RAID5 or ZRAID on them, same as if they were internal. You can potentially be bandwidth-limited since USB 3.0 has a 5 Gbps speed limit, but realistically only for reads and you’re still fine in terms of overall performance since they’re all spinning disks anyhow and 5Gbps is fast enough for any media server/NAS unless you’ve got a 10-gig LAN/internet connection and feel the compulsive need to saturate it.