Apple crippled Disk Utility in El Capitan, in their usual name of making good functional things pretty & pretty useless.
Luckily I’m far from the first person to need to create RAID and/or encrypted CoreStorage volumes, in El Capitan. Florian Knapp has a concise summary of how to set up an encrypted RAID volume. Tom Nelson (of About.com) has a slightly more detailed tutorial for managing the RAID part.
Now I just wish the hard drive industry would actually push capacities up, like they once did, so that I don’t have to resort to striped RAID sets just to make a disk big enough for Time Machine backups. It feels like we’ve been effectively stuck at 6 TB for many years now, and affordable 8+ TB drives aren’t really on the horizon (Seagate & Western Digital have offerings, but historically have been bad brands for drive reliability, e.g. Backblaze’s data, plus my own personal experience with their drives).
Update: macOS Sierra partially restores Disk Utility’s functionality, though not enough to be useful. It adds a “RAID Assistant” which lets you create unencrypted RAID volumes. The core Disk Utility app can also initiate manual repair of RAID mirrors, and delete RAID volumes.
It’s something of a mystery why you cannot create encrypted RAID volumes with the RAID Assistant. It doesn’t offer any encrypted file systems as initialisation options, and attempting to erase the unencrypted RAID volume in Disk Utility, to replace it with an encrypted version, fails with the bullshit error message:
An internal state error occurred Operation failed…
Furthermore, encrypted RAID volumes (or more precisely, any RAID volume that’s part of a CoreStorage Logical Volume Group) don’t get recognised as RAID volumes in Sierra’s Disk Utility unless you connect the underlying drives while Disk Utility is running. Even then it’s hit or miss whether it’ll correctly recognise not just that it is a RAID set but also that there’s an encrypted CoreStorage volume on the set. And I’m not even going to try testing if it can actually repair a RAID mirror in that configuration.
To be clear, RAID volumes that don’t have CoreStorage volumes atop them seem to work fine. It’s evident that Apple simply don’t support encrypted RAID volumes. Maybe in next year’s macOS – it must be hard adding support for things you already fucking supported until you pointlessly removed support for it.
FWIW, here’s a howto from Macworld on how to use the new RAID Assistant, if encryption isn’t something you want.