How do I stop apt looking for the Mint USB?

That error points to a leftover installer-media entry, not a general storage problem. Open Software Sources and disable or remove the entry beginning with cdrom:. Then refresh the package cache or run apt update again.

You can also inspect the configured sources and comment out the CD-ROM line, likely in Mint’s official repository list. Leave the Ubuntu and Mint network sources enabled. After saving the change, run sudo apt update.

APT is trying to use the installation image as a package source. You could reconnect and mount that media, but for getting current packages, disabling the cdrom source is the simpler fix. The error message is about that source, not proof that your persistent storage is full.