(I searched but didn't find this issue addressed)

I'm attempting to upgrade a friend traveling from RST via MSP to PHX using my One Pass miles.

RST-MSP is on Mesaba on a one-class aircraft, while MSP-PHX is mainline NW metal. Thus, I'm only interested in the MSP-PHX segment.

Speaking with a CO agent, I was told that while there are mileage upgrade seats available on MSP-PHX, the upgrade was not possible due to the Mesaba segment's presence on the reservation.

According to CO (actually the third CO agent I spoke with), this is a recognized flaw in their relationship with NW. Even though I have no interest in upgrading my friend on the Mesaba segment (it's not even possible since it's an ERJ), the Mesaba segment is preventing the mileage upgrade from being processed.

Does this sound correct to you folks? Does Mesaba not play nice with NW and CO?

For all intensive purposes Mesaba IS NW. There is no Mesaba ticketing system - just NW's - there is nothing wrong with the relationship. I'd try another call to CO, or a call to NW to see if its possible on their end before calling CO back (not possible for them to do it for you, since you need to do it with CO miles, but if they can do it IN GENERAL if someone was using NW miles)

Also, if your friends ticket is on Mesaba, its on a Saab 340 Turboprop, as that's the only plane Mesaba flies now - the ARJ's are gone (Avro Regional Jet with 16 FC), the CRJ's are Pinnacle, and Mesaba has the Saabs, the beautiful slow loud cramped saabs.