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You could go into your BIOS an see if the machine supports booting from the 'USB'. Another possibility would be to use a 'Hot Key', possibly 'F12' or 'F11' to select alternate boot device.
Some BIOS support a boot 'other' option within the boot setup.
Another option would be to use the 'sbootmgr
' from a floppy device.
This link and others are available from 'Slackware-Links
'. More than just SlackwareŽ links!