|Every hard disk has a Master Boot Record, not just boot disks. The MBR on a boot disk does contain extra information that is not present with a non-bootable disk. |
If you by chance did manager to remove the MBR it would render the disk so useless it would have to be repartitioned and reformatted. The MBR is placed on the disk when the disk is partitioned and it is the MBR that contains the partitioning information. Every hard disk is partitioned even it is just one single partition for the whole disk.
If the USB doesn't work I would try some data restore software. If you have been messing about with the MBR you may have already done some irreparable damage.