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to clean/wipe mbr...including partition table....
dd if=/dev/zero of=/dev/sd(x) bs=512 count=1
as root...My flashdrive usually uses /dev/sdd, check yours first!
sudo dd if=/dev/zero of=/dev/sdd bs=512 count=1
"NOT sda1, for flashdrives type mount and note the usb device info.
unmount the device and check you have the correct info, it should be gone.
Remount the device and continue."
Hope this helps, regards Glenn