musical chairs

hello, 

i have a disk with data on it /dev/hdc, 
    and a brand new empty disk /dev/hdb. 

want to set up raid 1 without moving data off hdc. 
    it very large and i have no where to put it

will this work?: (notice # of devices)

mdadm --create --verbose /dev/md0 --level=1 
        --raid-devices=1  /dev/hdb

will this create an unmirrored raid 1 setup? 
    can i start and mount md0?
    then copy the data from hdb to md0?

then?:

mdadm /dev/md0 --add /dev/hdc


seems too good to be true, 
    maybe you know another way?

jd