win8.1 functioning system to move to VirtBox.
 

is possible to copy an existing fully operational win8.1 system to a virtualbox partition and will after it is copied, will it work?

I have a two HD desktop with SDA as debian and SDB as a bootable windows system using the multiboot grub 2 software on the MBR.

Let me re-explain.....

I have a desktop that dual boots debian on SDA and Win8.1 on SDB .Each drive is 1tb.. ( Lots of space)

If I were to backup fully the sdb drive ( win8.1) to my NAS...and setup SDB as virtual box and then restore the win8.1 drive image will it work and a virtual win8.1 machine.

I think this is a Windows 8 question not a VirtualBox/Linux question.
What you're effectively asking is "if I move a Windows 8 install from one machine to another will it work". My initial thoguhts would be that it won't because, for example, if the install was done on a Secureboot machine the kernel won't load as it has to be booted from EUFI with secureboot. However, if it does boot you might be able, then, to unload and load the relevant drivers to get it working.

this is being reloaded from the same machine and not using eufi and secure boot....I do have the win 8.1 disk but would like to avoid reloading the software...

If you're not using EUFI or secure boot it may boot up but you will still have to load and unload the relevant drivers. VirtualBox is a virtual PC with certain hardware which will likely differ from the machine it is running on.

Past versions of Windows would BSOD upon boot with a major hardware change, such as a motherboard. Effectively you're doing the same here, and I would expect Windows not to boot at all.

No.

There are some P2V apps that can convert some windows OS's and change the hardware (HAL) as it does it and even fix the authentication.

As noted above, a VM is not a real computer and does not use your native hardware to decide what type of machine it is running on. MS uses hardware (and linux used to) at boot. You can add in drivers before you move a windows system. The problem then becomes if you have a OEM or retail version of Windows. If you have an OEM the authentication or authenticity is usually tied to bios. You can't easily take your current bios and make it available for a VM.
If you have a retail copy of W8 then you could do some type of backup and restore with system state. That tends to work.

Thanks everyone.....guess I will install VBox and reinstall win 8.1 and the other software I have on that O/S.

You may not need to re-install it. A backup with system over a clean install should work but I have not tested it. Microsoft technet pages would/should tell you how to do that.