What do you mean by this is this a separate usb with the slackware iso file copied to it or a slackware tree with a slackware directory with all the subdirectories containing a through y subdirectories?
I also tried mounting another usb with the media tree on it
If your using a usb key to boot the installation setup from and it is larger than 2gb you should be able to copy the slackware tree to it. Then you will have to create a new directory to mount the usb key to before running setup. for things to work. Do not mount the usb key to /mnt or a /mnt subdirectory. Then use the premounted directory option to install from.