Right-click WinUAE and select "run as Administrator". That's the only way to get full admin privileges.
Also disable your antivirus software as it might protect the CF card from being written to.
Did you clean the CF card before you started? Run DiskPart, list disk, select disk, clean, exit. If WinUAE does no longer recognise the card now, run DiskPart again, list disk, select disk, create part primary, delete part, exit.
After each change safely remove the CF card and reinsert it.
