I have a Barracuda Spam Firewall appliance, with a failed software RAID set. It clearly notes that it is running in degraded mode during the startup. I confirmed that one of the drives in the set has failed using the HDD vendor's drive diag utility.

So I removed the drive and put in another drive of the same vendor and capacity and the RAID set is still degraded. I have booted into the BASH shell and the Linux distro they base the device upon appears to be a rather old distro, it appears that it is based on the 2.0 kernel. The hardware is pretty old so I am not entirely surprised that the kernel is ancient, but I am not terribly concerned about the kernel version as much as rebuilding the RAID array.

I tried running mdadm, but the utility doesn't seem to run for some reason. I am not clear whether such an old kernel would require a different command to rebuild the array than a more current version of the kernel, but I figured I would inquire anyways.

Need a lot more details about mdadm 'doesn't run'. Please show exact cmds & msgs.
Also
uname -a

and any info you can get eg /etc/issue.net to establish which distro this is based on.
Also,

fdisk -l

All of the above as root user.