virtual box resize error when resized to 500gb

[while-one]

I am on Ubuntu 12.04 as host. Also have Ubuntu 12.04 as guest. The guest is setup as dynamically allocated virtual disk with 20gb limit. So when disk became full, need to resize it. Used the below commands to resize to 500gb

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    VBoxManage clonehd "source.vmdk" "cloned.vdi" --format vdi
    VBoxManage modifyhd "cloned.vdi" --resize 512000
    VBoxManage clonehd "cloned.vdi" "resized.vmdk" --format vmdk

There is enough space on hard disk. But when i resized to 500gb and import the resized.vmdk file,not able to boot and a grub rescue screen appears with this message

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    error: unknown filesystem.
    grub rescue >

The output from grub rescue

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    ls /
    ls (hd0,2)/boot/
    ./ ../

I then tried to resize to 200gb. This works. Able to boot.
Is there any way that i could increase the disk size to 500gb.

Thanks in advance

[mpack]

It's ironic that you identify guest and host OS version - neither of which is directly relevant to your question - but fail to give the VirtualBox version, which is very important information.

And I'm curious: why all the juggling to keep the disk in VMDK format? Why not keep in it VDI format, which is after all the native VirtualBox format?

However, in answer to your question: if you are using a recent official VirtualBox release, and it gave no error when cloning or resizing, then the resize to 500GB undoubtedly worked as far as the disk itself is concerned. If the guest failed to boot then it's more likely to be something about grub or the guest OS which doesn't like the jump to 500GB. E.g. perhaps this crosses a threshold which would require a different disk format, e.g. larger block size etc.