Very interesting as when you create an RAID1 under Linux with mdadm command you get the following hint for the bootloader:
mdadm: Note: this array has metadata at the start and may not be suitable as a boot device. If you plan to store '/boot' on this device please ensure that your boot-loader understands md/v1.x metadata, or use --metadata=0.90
So maybe nanoboot does create the RAID in a different way which isn't supported in DSM 5.1 anymore or uses an outdated version of madam.
Or the kernel is too old.
[root@localhost ~]# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1
mdadm: Note: this array has metadata at the start and
may not be suitable as a boot device. If you plan to
store '/boot' on this device please ensure that
your boot-loader understands md/v1.x metadata, or use
--metadata=0.90
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
[root@localhost ~]#
More Infos can be found here for example:
http://www.thomas-krenn.com/de/wiki/Linux_Software_RAID
