Cloned VirtualBox CentOS disk is unbootable after cloning in GParted

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http://unix.stackexchange.com – I cloned and enlarged an existing virtual CentOS-6 disk by the following steps: Create 15GB virtual disk and mount the old and the new disks in GParted. In GParted copy boot, swap and ext3 partitions from the old disk to the new one. Mark the new boot partition with boot flag. Resize the new ext3 to bigger size. I disconnected the old disk and connected the new one. Now I cannot boot the VirtualBox. I get a black screen. What seems to be the problem here? Thanks. (HowTos)