snip

Well once you have partitioned the new disk you should be able to just do:

mdadm --add /dev/md0 /dev/sdX1 (whatever X is for the new drive).

Then /proc/mdstat should show it rebuilding the raid.

Result:

1. partitioned the drive using gparted (in the running system) to be sdf1

I did a boot into a rescue disk to check but it had worked on the running system. This was not listed as possible but the command worked!)

2. read and read (a few times) the man page for mdadm

I was looking specifically at the commands -D -v --add as suggested by William

found the total lack of examples frustrating (a normal man page I think!)

3. took the command above, suggested by Len, and changed it somewhat to
# mdadm -v --add /dev/md0 /dev/sdf1

4. checked the rebuilding by using

# vi /proc/mdstat

Thank you to those who contributed!!

Dee