Sorry to hear of your troubles setting up RAID.

Unfortunately, I don't think I'm going to be much help to you.

I've never set up software RAID (actually I've never set up any sort of RAID). It's always been something that I've intended to play with, but never had the need (I've always just used LVM and relied on backups).

Also, I have a personal preference for the commandline, so don't have a ton of experience with Webmin. I use Proxmox as a Hypervisor and run everything as VMs or containers, so have very little experience with Linux on bare metal too.

Having said all that, it should certainly be possible. Under the hood, TurnKey is Debian. v14.x is based on Debian Jessie and our upcoming (and well overdue) v15.0 release will be based on Debian Stretch. We build the Webmin packages ourselves, but other than our TKLBAM module, all the code comes from Webmin themselves.

I see that you've installed mdadm. AFAIK that should be the only dependency/requirement.

Out of interest though, your request for assistance has actually assisted me to discover a bug in our upcoming v15.0! So thanks for that! (FWIW to you...).

I tried following the tutorial you linked to myself (using a v14.2 VM I had handy). I think I may have missed something, but it won't work for me either. However, my experience sounds a little different to yours. I got as far as the "RAID device options" screen but I'm only seeing one "Partitions in RAID" (and the same one showing under "Spare partitions"). If I try to continue, then I get an error "Failed to create RAID : At least 2 partitions must be selected for mirroring". So that's a bit of a fail...!

I really need to get back to v15.0 development so we can push it out the door, so I can't really afford to spend any more time on this right now sorry. The only other thing that I could suggest, is trying from the commandline. One of the (many) reasons why I prefer the commandline is that unless you watch a video, I find that it's quite easy to misinterpret things, miss vital steps, or click the wrong thing when using a GUI. I find using commandline tutorials is generally more reliable as IMO it's harder to misinterpret things and often error messages will give clear signs on anything that isn't working as it should (and often point to why, or at least give an error message to google). It's also much easier to find out exactly what the commands are actually doing (e.g. via the man pages, or even google). FWIW I did a quick google for you and found a tutorial that might be worth a try. It's for a previous version of Debian (Squeeze was the one before Jessie) but I would expect it to be similar, if not the same (as hinted above, anything that doesn't work, try googling explict error messages).

If you'd rather persist with Webmin, then perhaps consider posting on the Webmin support forums. They will probably be able to help you out much more than me. Please note that the Webmin theme we provide by default is different to what Webmin use by default (although we'll be switching to the default theme for v15.0) and is an older version, but otherwise, everything should be as expected. As I noted above, the basis of TurnKey v14.x is Debian Jessie.

Sorry, I couldn't just guide you to the exact steps required, but hopefully I've been of some assistance. Good luck with it and if you manage to work it out, please post back as I'm sure other users will find it useful.