how do I connect a firewire 400 or 800 to mac mini 2018

Hello

In order to connect a firewire 400 or 800 to your 2018 Mac mini you would need to look into purchasing an adapter that is usb-a to firewire. This isn't available on Apple website but feel free to check with any other third party company.

Hope this helps

You need a Thunderbolt adapter

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

followed by a FireWire adapter

Apple Thunderbolt to FireWire Adapter - Apple

Adapt it to Thunderbolt 1/2 and then Thunderbolt 3.

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