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#11
It ALWAYS happens if you modify the starting sector of OS partition. Or you modify disk signature of disk (all disks have a unique disk signature). This because the bootmenu entry points to those values. If you display bcd output using bcdedit it's automatically translated to drive letter or harddisknummer/partitionnumber if drive letter is unknown. But if it's pointing to old none existing partition it's listed as "unknown".
How to use Boot Configuration Data Store Editor (BCDEdit) to manage BCD stores