How do i delete a folder written on a rewritable disc

Jonmor68

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Even when you use a drag and drop system that allows you to treat a cd as a floppy, deleting files only makes them appear to be gone, but does not regain the space used by those files.
I find rewritables notoriously unreliable regardless of make, I use write once media for stuff I want to keep and usb stick for temporary holding to move around.