MD Raid Devices
If you're getting
mdadm: cannot open /dev/sdb1: Device or resource busy or something like that chances are is that the array isn't created correctly. You will need to stop the array with
mdadm --stop /dev/md* and recreate it.
The rest of the troubleshooting commands are for scanning and re-assembling arrays for example you put the disks in the wrong order. It happens to everyone at least once :)
Creating an Array
You can make an array a lot of different ways, the current best way is with LVM & DMRAID; however there are A LOT of cases where MDRaid is better to use. A good example is with compatibly with hardware controllers. Dell's PERC orders and creates it's discs in a way that is compatible with importing and exporting via Linux MD devices.
This example creates a MD device
/dev/md127 the level is stripe meaning it is striping the data aka RAID0. The
chunk are the chunk aka strip size and the number of disks in the array. In this case
/dev/sdb1 /dev/sdc1 are the two disks in the example. You can configure this as needed