Anyway to access file from unmounted disk?

My boot drive "Macintosh HD" suddenly got full and now it won't boot or mount and I need to access a folder and delete files to free up space. I'm now booting from an external bootable drive.

TechTool Pro 8 and Disk Utility can't mount or repair.

Terminal shows the unmounted drive so maybe I can access the Downloads folder and delete files but I don't know the key prompts to get me there.

Any suggestions?

Thanks

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