If you ever need to switch back to an MBR partition scheme from GPT, for whatever reason, cfdisk will spit out an error saying that the disk still has a GPT partition on it.
This is from a GPT partition scheme written at the end of the disk.
To erase this and restore functionality back to MBR style partitions use these commands. This works with any distribution of Linux with an install Disk, not just Slackware, provided the disk does in fact have gnu parted installed on it.
You'll be asked some questions shortly afterwards just type yes and ignore and afterwards reboot the media.