How to allow MBR changes when installing to HDD?

Hi,

I have a laptop without floppy and cd-rom drive. Now I wanted to use Parallels to install MS DOS on the HDD which is connected via USB.

So far so good, but when MS DOS install process wants to write to the MBR I get the message, that this write process was prevented because of security reason.

Is there a way to disable this security feature?

I know what I am doing and would like to install MS DOS this way on my harddrive.

Thanks for help!