If you want to manage a software raid-1 under debian after a disk failure you have to
- remove the failed disk from md-device
- change the failed disk with a new one
- copy over the partition table from the remaining good disk
- add the new disk to the md-device
- install grub on the new disk
If sda failed in your md0-device, sdb is ok and there is only one partition on the md-raid, this would look the following.
mdadm --manage /dev/md0 --remove /dev/sda1 # change failed disk with new one sfdisk -d /dev/sdb | sfdisk /dev/sda mdadm --manage /dev/md0 --add /dev/sda1 grub-install /dev/sda