Q:

Reconnect drive to raid-1 array with mdadm

I need a large and relatively robust backup method.  I had an old desktop running debian and put in two identical 2tb drives in raid-1 configuration using mdadm
So I would start the array using 
sudo mdadm --assemble /dev/md0  /dev/sdb /dev/sdc

That was working well.  Then I decided that I needed to make sure I understood how I would deal with it if a drive failed.  So I unplugged the sata cable from the drive in /dev/sdb and I started the array with 
sudo mdadm -assemble /dev/md0 /dev/sdc --run

I confirmed that it seemed to work as expected.  I did not alter any files on the drive.  Then I replugged the sata cable.
Now when I try to start the array with both drives:
sudo mdadm --assemble /dev/md0  /dev/sdb /dev/sdc

I get the following.
mdadm: /dev/md0 has been started with 1 drive (out of 2).
When I try
sudo mdadm /dev/md0 --re-add /dev/sdb

I get mdadm: --re-add for /dev/sdb to /dev/md0 is not possible
How can I most simply bring the array back online with both disks?  I would prefer not to rebuild the entire disk if possible and I would prefer not to loose the data on /dev/dsb.
Any help would be greatly appreciated!

A:

"I need a large and relatively robust backup method."

RAID is not a backup method.

"How can I most simply bring the array back online with both disks? I would prefer not to rebuild the entire disk if possible and I would prefer not to loose the data on /dev/dsb."

You will have to rebuild the array.
By using the array with one disk disconnected, they have become desynchronized. With appropriate metadata, the resynchronisation can be reasonably fast. But in any case, in order to resynchronize the desynchronized disks, a rebuild must be done. 

