[Help] iMac cannot download any files due to storage space, but over 450 GB is available

Hello all,

I am trying to download videos and images onto my Mac, both from an external drive and from the internet. When I try to download from the external drive, I get this error message:

"Operation Stopped. There is not enough available space to continue the file writing. Make room by deleting existing videos or photos."

When I try to download from the internet (via Firefox), I get this error message:

"There is not enough room on the disk to save [file]. Remove unnecessary files from the disk and try again, or try saving in a different location."

So basically, the problem seems to be that I do not have enough storage space on the computer. However, going to "About this Mac" and then "Storage," I see that there is 467 GB available of 1 TB. I cannot download even small images without encountering the same storage space error. I have tried clearing cache (which was over 100 GB), and I have deleted every file that I could (and emptied the trash).

Even with over 450 GB storage space available, I cannot do anything. Copying already existing files works perfectly fine, which shows that the storage space really is there. I do not have time machine connected. I have tried restarting my iMac several times, with no difference.

Any and all help in fixing this issue would be greatly appreciated, for if I am not able to download even small files onto this iMac, it will remain essentially useless. Thank you all for your time.

Note: I would normally attach screenshots of the error message and my storage space, but it seems that there is not even enough storage left for screenshots, as new ones automatically vanish.