[weblacky]

Hmm, that’s interesting. I used VirtualCloneDrive in windows and I could view its contents. The iso is a trusted one too, as it came from fsck. I assumed the steps were to download the zip, extract with 7zip to the sd card, the end.

From your description I'd say you downloaded the wrong thing. Going on the link you provided, which does not point to install media, but looking at the website, when you download an ISO it remains an iso. There is no zipped iso media so I assume what you did if you downloaded a media "tarball", you need that if you're doing a network based install which is file fetching based-install and the firmware isn't reading a disk. That website should be just fine but assuming you're downloading something like this: 

https://fsck.technology/software/Silicon...%206.5.21/

You'll notice there's nothing to unzip, they are media images and are treated as such. If you're using an emulated CDOM install or a real CDROM install you need the raw images!  If you're trying to put the images on a Zulu you literally put the image file on the Zulu, there's nothing to unpack, nothing to extract, you want the raw image as a file.  Yes you might need to name it with an 512_filename extension to make sure it's treated correctly, but to you it's a 600MB blob of data, a single file per CD image.

If you're doing a network install or a live OS install from a working system you need the files more than you need the media. The reason they exist is as I described above. The CD images are normally not readable by anyone outside of Linux or Irix... So people often need the files to do the network install and they don't want to spend the time cracking open the modified CDROM Disk label EFS Drive images. So often time, sites provide both as a shortcut.

The only thing I'm trying to impress upon you is that the CD images are supposed to be absolutely unintelligible junk to anything that can't read an EFS drive image. So in Windows/MacOS you should not be able to mount them, you should not get a blank image, you should literally get an error that says this is not a CDROM image, which would be accurate.

When this was all relevant/recent you were supposed to own an SGI so you could obviously read your own install discs and copy the files up using a network connection into a file server in order to prepare a network installation source. It just worked itself out if you owned everything. Now people don't have that so often times archive sides provide both, hence your confusion.

On an SGI Indy, Irix 6.5.22 is what you want, try WINWORLD: https://winworldpc.com/product/irix/6522

Also in case no one has told you 6.5.22 is the last SGI irix to support the Indy as well as well as Indigo2.  So please don't try to install a version of the OS of higher than 6.5.22 as it will literally crash and not work. 6.5.22 was a great release, there was nothing wrong with it, some people even run it on later hardware, this is the one you want so please aim for 6.5.22.