if you want to mount a raid-1 member in a external system, you should install mdadm and checkout those commands:

# Connect harddisk to auto-plugable controller/system
# Check which device name was assigned to the pluged-in harddisk
tail /var/log/syslog
fdisk -l /dev/sdb
mkdir /mnt/data
mdadm --examine /dev/sdb3
mdadm -A -R /dev/md0 /dev/sdb3
mount -o ro /dev/md0 /mnt/data
# Read, what you want to read ;)
umount /mnt/data
mdadm -S /dev/md0
# Remove the harddisk

that is it. thnx Milosz for your help and saving my time 😉