Mac Pro 3,1 (Early 2008) Booting Woes

Hi all,

I could really use some help from this knowledgable community to get an old MacPro 3,1 back up and running.

Basically for a bit of background, our IT department at work we’re having an equipment clear out and I managed to bag a Mac Pro 3,1 without any drives with the intention of setting it up as a perfectly good home Mac/Windows machine. I will try to be as descriptive as possible but please bear with me.

The computer in question has not been used in at least 5 years so my first port of call was to give it a good clean out and check components, reseat risers etc - all good.

Next, I took a fresh HDD and installed a copy of El Capitan on it using another Mac. The drive boots up and works well in my external drive caddy - all good.

I put the HDD inside the Mac. Boot screen appears (albeit with some artefacts on the screen so I know the 8800GT is on its way out but still displays an image) and the loading bar gets to about 50% and hangs. I leave it for half an hour or so and nothing happens - odd. I carry out an SMC, PRAM and RTC reset (new battery for the latter). Still nothing.

I check the status lights on the logic board and the RAM risers. Nothing unusual there.

I try to install El Capitan on a fresh SSD (as before) - same result.

I do the same with a fresh copy of OSX Lion on an HDD and and SSD - hangs in both cases but then continues to boot loop (powering up, getting to 50% or so on the loading screen and restarting)

Try to boot the Mac directly from the installer USB - same result with boot looping.

EDIT: Tried recovery mode and safe mode - same hanging

Try with different RAM and riser combos (taking it back to 2GB, I have 6 sticks in total equal to 8GB) - same result every time.

I shove in an old Windows10 drive - boots and works like a charm with no bootlooping or restarting issues, really odd!?

I can load single user mode and Verbose in terms of Mac things, but apart from that the only drive that’ll boot is one with windows on. The only conceivable error I can see (and even then I have to admit I don’t know enough about Mac OS to know if it is one) is kSMCKeyNotFound when using Verbose.

I’d really appreciate any advice that anybody can give to give this machine a new lease of life. It’s great I can get into windows but I don’t know why OS X will just continue to boot loop.

Any help is gratefully received :)

Thank you!