How to mounte a raid1
 

I have disk which previously has been part of a /dev/md1 raid1.
Now I have moved the disk to another computer with another boot disk and want to mount it so I can copy some file. My raid partitation is on /dev/sdb1.
What shall I write? Shall I do an assembling first like
mdadm --assemble /dev/md1 /dev/sdb1

Yes, that is exactly what you'll have to do. Add the "--run" parameter to ensure the device starts even though one array member is missing.