I installed Ubuntu 18.04 as a second OS on a computer which had Windows 7 already installed. The computer had an Intel RST FakeRAID level 5. After a good deal of messing around, I finally followed the instructions in the top answer to this question. This worked fine... until one of my hard drives failed.
I replaced the drive, and booted into Windows, where I rebuilt the RAID volume with Intel Rapid Storage Technology.
Now Windows boots and functions perfectly fine, but Ubuntu hangs while booting.
What's going on, and how can I fix this? According to this page, mdadm is supposed to be able to handle Intel RST.