SilentDecode

I Googled with Bing

You what now?!

Try to use the APC Firmware updater. Your firmware might be stuck and the way to go is to update the firmware. This might be a little hard to find, because APC recently did a dick-move and threw all their NMC firmware utilities behind a MASSIVE paywall. If you really can't find the tool, I can link you one. Send a DM if you're interesed. I think I have a tool for the AP/9630/AP9631, but I'll have to check to be sure.

is there a good solution for this?

Get a ODD caddy to HDD adapter, put in a proper quality SATA SSD and use the cableing of the ODD to connect it all. Install the OS and boot from it. I've been doing this to many servers over the years, also in my current R730. Works splendidly.

I was thinking of using a PCIE m.2 adatpter

The R720 and newer R730 do not support boot from NVMe. So without some Clover bootloader magic, you can't boot from this.

I am also running into problems with the lifecycle controller

You can update firmware via iDRAC itself. If you can access iDRAC with a web browser, just upload the Windows .exe file in the 'file upload' section in iDRAC.

Oh, and if your firmware of your iDRAC is still on 1.x.x.x, DO NOT UPGRADE STRAIGHT TO THE NEWEST VERSION OF 2.x.x.x. This will brick the controller. Just take your time with updates and just go slowly through every other firmware update available, until you're on the latest update. I really can't emphasize this enough.

but it could be a fun proof of concept?

Having gigantic clusters of small ARM computers, has been around almost as long as ARM is on consumer hardware. Definitly since the very first Pi 1. Loads of people have constructed many different configurations with all kind of different hardware. Loads of people here even run K8s and other stuff on their ARM cluster.

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Tell me why I shouldn't do this!

If you have no use for it, why bother? Only if you want to learn from it, it can be useful. But still, you really have to think about what you want to do with it.

Oh, and power reasons. If you're in the USA, and your power is cheap, then why not. If you're not so lucky with the powerbill, then just get one big system (x64 or ARM) and just run stuff on there.

Nope. You need to buy a seperate case for this and throw everything over.