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I have an efi-based 64 bit system with win 8 pre-installed. I want to downgrade to 32-bit win 7 and so I booted up a bootable win 7 usb and opened command-prompt with shift+f10. I used convert mbr command on disk 0 (which should be gpt) and it says: "the specified disk is not convertible. cdroms and dvds are examples of disks that are not convertible"

Now, I have not cleaned the disk yet, so this might explain the error. However, I would think a different error message would appear if that were the case and I don't want a laptop without an os. Furthermore, I cannot enter "Repair your computer" as it said system recovery options was incompatible. What is going on here? i definitely want to convert to mbr for other reasons too.

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You have to remove all partitions. This a limitation of Windows’ disk management.

After that, you can use convert mbr. But since you have to remove all partitions anyway, clean is much faster and removes the partitioning scheme, too.

And yes, this is the exact error message you’ll get on a non-empty disk:

DISKPART> list partition

  Partition ###  Type              Size     Offset
  -------------  ----------------  -------  -------
  Partition 1    Reserved            32 MB    17 KB
  Partition 2    Primary           2014 MB    32 MB

DISKPART> convert mbr

Virtual Disk Service error:
The specified disk is not convertible. CDROMs and DVDs
are examples of disks that are not convertable.

On the other hand, if the disk is empty:

DISKPART> list partition

There are no partitions on this disk to show.

DISKPART> convert mbr

DiskPart successfully converted the selected disk to MBR format.

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