RAID 1 and a spare?

I’m upgrading from 2 “My Book Live Duo”, a few “3TB My Book USB3” and some HD from a desktop that died.

My new PR 4100 has 2 x 8TB drives set up as RAID 1. What I would like to do is use one of the spare bays to install random drives that already contain data and copy that data to the RAID 1 volume.

Is this possible, what RAID mode would I use, and would I loose and data on the current RAID 1 by changing modes? BTW, I use a mac if that matters. Thanks!

Yes it is possible.
Create your 2 disk RAID1 volume first.
Insert a 3rd disk with data on it.
Use SSH and do this

Show file systems

df -h

If your 3rd disk is not mounted yet, you could do this (assuming data on 1st partition of /dev/sdc)

mkdir -p /mnt/somedisk
mount /dev/sdc1 /mnt/somedisk

Copy your data e.g. to your Public share

rsync -a --progress /mnt/somedisk /shares/Public/backup

Finally umount your disk

umount /mnt/somedisk

Ensure all /dev/sdc partitions are unmounted


And remove the disk again.