Find out first if they are sata or pata(ide)drives. You'll have to open the case to see the connections. Forget the external USB backup drive as that is what it is whether you have images or not. If you have two physical hard drives and not one partitioned into two, then I'd leave the data HDD (the one without the OS), alone. Buy a new HDD and connect via a caddy (my preferred way) and use Acronis to clone the C: drive. If the D:\ drive is a partition of a single hard drive then do the same again but clone the whole drive, partitions included.

I use Paragon so I can't advise how to do it with Acronis. Hopefully someone here can point the way of how to 'clone' with Acronis.

lotvic , why not simply clone to usb drive? Supposing C:\ and D:\ drives are partitions? If not and both are two separate drives then the data drive need not be touched.

There are usually several different ways to choose from to achieve the end result depending on the level of experience, Hardware equipment, software for cloning & backing up, types of backups already created etc.

Better to give the OP a choice. I have used the usb ext drive way in past and also the internal for both cloning and putting a backup image.tib on. With any method you have to be careful that you don't put it on the wrong drive and lose data you didn't want to :)