Reuniting disks in a raid1 array

[Frédéric Marchal <frederic.marchal@wowtechnology.com>]

Hello,

I had two disks in a mdadm software raid1 on an old computer.

I moved sda to a new computer. It ran there in a degraded raid1 for months.

The second disk, sdb, kept running as the lone survivor of the original raid1 
on the old computer.

Now, it's time for the old computer to retire and sdb to join its partner in 
the new raid1 on the new computer.

How can I do this safely?

Do I have to wipe sdb before adding it to the new computer? If so, how do I 
make sure raid data is gone from every one of the three partitions?

Or maybe mdadm will see that sda was used more recently than sdb and will 
synchronize sda onto sdb?

Thanks,

Frederic