Well, this explains it

After calling SGI and asking what the last supported version of IRIX would be for an Indigo2 workstation, I spent an exorbitant amount of money for a secondhand set of CDs on ebay. OK, not exorbitant compared to the $600 retail price of IRIX, but still more than I’ve ever spent on Operating System software ever.

I did research and tinkered and did more research and more tinkering and finally today I get the answer to why my installs keep failing:

http://www.sgi.com/products/software/irix/releases/irix6526.html

Changes to Support Mode Status
The following workstations, graphics and scalable server systems have transitioned to retired support mode for the IRIXÂ® 6.5.23 release. IRIX 6.5.23 and beyond do not support installation on these systems.

ChallengeÂ® S, ChallengeÂ® M, ChallengeÂ® DM, ChallengeÂ® L, ChallengeÂ® XL
IndigoÂ®, Indigo2â¢, Indigo2â¢ Extreme, Indigo2 Impactâ¢, IndyÂ®
Power Challengeâ¢, Power Indigo2â¢, Power Onyxâ¢
OnyxÂ®

Yep, just plain unsupported.

Now I guess I get to look for a copy of IRIX 6.5.22. Maybe someone wants to trade for 6.5.26.