imho mkcd is a poor way to write directly to a CD/DVD. From memory, the option I used is
-S (I only use nim these days having become lazy).
I have not used AIX 4.2 for a long long time. I think AIX 4.3.3 already had mkcd, but unsure of AIX 4.2.
For these "ancient" bare metal systems, if you lack an internal tape drive I recommend appropriating an external SCSI tape drive.
Assuming you have more than just rootvg - also make regular
savevg backups of the other volume groups. The data may be more important than the rootvg.
I also recommend you glance at
http://www.rootvg.net/content/view/134/88/ for getting something bootable. Sometimes it is only the booting that is failing.