Developer and Support Tech
That's not an error that stops all operations, it's a notification. You can continue all operations normally. However, because the MBR is THE most important sector on a disk, this message is displayed as soon as DiskPatch sees it can't be read. Essentially, no repairs are possible if the MBR is not accessible for reading or writing. This means that, to get data from this disk, you will either have to clone it, or you can use iRecover to copy files from the disk.
Another possible solution might be to force DiskPatch to try and write to the MBR, to see if the sector can be re-allocated (replaced by a good sector from the spare pool). You can try this yourself by simply trying to wipe the MBR a few times, or by running a read/write surface scan. If the MBR is replaced and in working order again, a normal repair can follow (if the disk isn't in bad shape; your other post suggests that something's quite wrong with this disk, so you may want to start by running a disk health check).
If you decide to try this, make sure to read the section in the manual that explains the (read/write) surface scan.