In order to try out new Software or Configurations I have found it useful to
setup a system using RAID1
then when I need try out new config or software I remove one of the drives
an experiment!

*** BTW: I've done this at least a HUNDRED times in the past ***

for the last 2-3 years I've used the following procedure

use SFDISK to duplicate partitions

ie. sfdisk -d /dev/sda | sfdisk /dev/sdb

cat /proc/mdstat check the status

mdadm /dev/md0 --add /dev/sdb2

cat /proc/mdstat check the status

mdadm /dev/md1 --add /dev/sdb3

cat /proc/mdstat check the status

Whilst its not necessary just like to get a feedback re: status with cat

NOW TODAY - its not gone well !!!

not once but 3 times ! - with 3 different source disks!

as soon as I add the new device

ie. mdadm /dev/md0 --add /dev/sdb2
and follow it IMMEDIATLY with cat /proc/mdstat - I find the % complete is
around 30% in about 2 secs !!
(these are 70Gb Partitions so I normally find it takes about 20mins)

if I immediatly type cat /proc/mdstat - its jumped to 80% or 100% shows
complete ie. UU

mdstat -D /dev/md0 - shows 2 devices & sync'd

HOWEVER... now BOTH the source disk and the new disk are unusable !

on the 2nd & 3rd attempt I made sure the SDA & SDB are correctly identified
cleared the new disk with Windows boot CD

Anyone have any experience of this or can tell me what I have done wrong