virtualbox.org

No, this time you seem to have found a suitable forum - at leasy as far as possible given that your problem is not with VirtualBox, you are failing to understand the Windows host filesystem.

I suggest that you open a new topic, and this time give full details of the command syntax you tried, the exact error message received, plus a screenshot as proof that the VDI file you want is present and correct in the folder path you typed.

I'll grab this old topic as I was facing similar problem - needed to burn linux host into hdd. I tried to avoid buying trueimage so I did following:
VBoxManage clonemedium --format RAW linux.vdi linux.img
then:
HDDRawCopy image into disk.
That's it.

You don't have to buy TrueImage - that was my recommendation in 2011, but this is now 2017 and things have moved on. Today I recommend downloading a copy of Macrium Reflect Free Edition. Acronis has become rather bloated and incomprehensible I'm afraid. Working with RAW images has not gotten any more attractive in the meantime.