When replacing a failed RAID1 member from the array, setting up the replacement disk requires to partition it exactly as the existing disk. But this takes time to do manually…
Fortunately, sfdisk comes in handy by providing a very fast shortcut:
sfdisk -d /dev/sda | sfdisk /dev/sdb
with sda being the source (existing RAID member) drive and sdb the target (empty) drive. Be absolutely sure which is which in your case before using that command, or you’ll lose data.