Make USB flash boot option. A few years back we use a floppy to store boot data. Now that floppy disk is not popular anymore, some system don't even have floppy drive anymore. So Slackware has made a usb device as a choice for alternative boot device. For a safety reason, I would recommend you to create one also. Just to make sure your Slackware can safely boot into the system later, if your computer support booting from usb device, please create a usb boot stick now. Remember that usb drive will be formatted and all the data will be gone. So save data in the usb drive first.