Check the BIOS settings.
If you're trying to boot from the Sandisk, you'll need to set the boot order to "D,A, SCSI" (or possibly "C,A, SCSI"). Some out-of-box IOpeners had this set to "C Only".
Alternatively, you can set the boot order to "SMARTBOOT", and select the Sandisk partition from within Smart Boot Manager. This is the same hidden menu you'd use to boot the IOpener from a CD-ROM if you have one connected.