Hello,
I installed CentOS on software RAID10 but made the mistake of installing /boot on /dev/sda instead of on md0. Now I fear that if sda fails my system won't be bootable. Is there a way to correct this?
I can't reinstall because we need the server online.

No way to change your install from a simple partition to a software raid or LVM setup without re-installing.

Depends on how md0 is assembled. Is sda1 part of the assembly? Then you have to re-create the array anyway. Because you are trying to use sda1 both as partition and as part of a RAID.

(I hope I interpreted your /dev/sda correctly as /dev/sda1)

I am not sure you can boot from a RAID10 array. Can you? Then you might move /boot to md0 and re-install grub.

Note however that you must install the grub boot sector on both devices of a RAID array order to be able to boot the second disk anyway. So having /boot on the md0 doesn't help a thing. You must also install the correct MBR on both disks.

So if /dev/sda1 is not part of the array, leave your /boot there and copy it to /dev/sdb1, and create an MBR on /dev/sdb

jlinkels