[clingbang]

I believe the magic lies somewhere in here :

By default, the emulated guest system receives the MAME hotkeys, instead of them being perceived by MAME as MAME key combinations. At least on MS-Windows, you can circumvent this behavior by hitting the 'scroll-lock' key while in and focusing on the MAME window, so the hotkeys reach MAME instead of the emulated guest. After that, you can press TAB to enter the internal MAME Interface and use the 'File Manager' option to select another CD. Just press 'scroll-lock' a second time to revert back to the original behavior.

Hope this helps.

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This was super, super helpful actually. Thank you! This enabled me to get things moving. Really appreciate it.

As for the problems I was having with mounting disks, I'm honestly not sure what the problem was. I finally got things working by just manually mounting the new drive via the MAME menu (which you so helpfully showed me how to locate) so that seems to be working ok. This is almost certainly a confluence of issues stemming from: I haven't used IRIX in 20 years, I haven't used MAME in ~10 years, and those two things combined make this pretty challenging!

That being said, my attempts to reinstall IRIX 6.5 were completely futile. No matter what I did it complained that the ISOs were not bootable, despite following peoples' instructions here. So hopefully the preexisting 6.5 that someone else posted upthread will suffice. Either way, thank you so much for the help.