Old firewire external drive to newer Mac mini via Firewire?

You need two adapters. A USB-C to Thunderbolt 2 adapter and then a Thunderbolt 2 to FireWire 800 adapter, to get from FireWire 800 to FireWire 400 you just need a simple 800 to 400 cable.

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

Apple Thunderbolt to FireWire Adapter - Apple

also see > https://www.youtube.com/watch?v=UW2Bbn_OjNA

Since your mini has TB2 ports all you should need is one of these - https://www.apple.com/shop/product/MD464LL/A/apple-thunderbolt-to-firewire-adapter?fnode=8b&fs=f%3Dadapter-rechargebattery%26fh%3D4595%252B4633%252B45b0

and a firewire 400 to 800 cable for the 400 devices. I connect a FireWire 800 OWC drive to my TB2 ports on my 2015 MacBook Pro with that Apple adapter and it works fine. And if I use a FireWire 400 to 800 cable and that adapter, even my original iSight camera will work with the MacBOok Pro.

I don’t know about the power error as that simply shouldn’t happen with an externally powered device, unless it is a problem with your adapter?

No need to apologize - I nearly didn’t post because I’d first read it the same and saw you’d answered.

OP, I wonder if the power error is because you’re trying to use a USB port? The FireWire power standards allow for higher voltage and a higher current than your USB 3 ports. I’ve never tried connecting FireWire to USB, so may be off base about that error. I’ve never had a problem going from FireWire 400 or 800 to TB2 or TB3 (my 2018 Mac mini) though.

Good catch Michael.

My bad, I thought we were going from a 2018 Mac Mini, until I re-re-read bighulawood's post.