iMac says Storage is full but I only used 10GB out of 1TB of storage...?
This happened to me once. I used Daisy Disk to figure out where the disk usage was coming from.

In my case, there was a problem with the coresymbolicationd cache, which was growing indefinitely (eating up hundreds of gigabytes).

After deleting /System/Library/Caches/com.apple.coresymbolicationd, everything returned to normal.