As given in the question there are two hard drives, one of 500 GB and another one of 1 TB. In which 500 GB drives is faster as compare to 1 TB hard drive because faster one is 3.0 and slower one is 2.0.

The reason of getting a warning is that you are running out of space in one of the drives in which you are copying the data of C: drive. There can be another reason that it can be possible that there is already some data on the hard drive so there was not enough space to add the large volume of C: drive.

To to be able to store data of C: drive we need to create another drive with the existing two hard drive. so let us suppose that the data of C: drive is 1200 GB so we have no option and we cannot have the whole data at one place.

So the drive with the 500 GB we call it drive A and the drive with 1 TB we call it drive B. so to be able to add/merge C: drive data we need to combine both the drives together into another drive, we call it drive X.

Drive A + Drive B = Drive X

(500 GB) (1 TB) (1.5 TB)

In the different operating system, there are different ways. In windows, you need to go under Disk Management setting and then you can merge two drives. In Linux, you need to use LVM to merge two drives.