Sometime our flash usb drive is error without knowing the cause. The master boot record (MBR) is damaged or corrupted.
We can recover the mbr by copying the mbr.bin available in slackware directory into the flash drive. This is amazing isn’t it. Slackware keep mbr record inside its original installation.
This is how to copy the mbr.bin to the flash. Sdb1 is the flashdrive node recognition by slackware, the location in my slackware 13.1 is in /usr/share/syslinux/mbr.bin
dd if=/usr/share/syslinux/mbr.bin of=/dev/sdb1