|It depends on how you made the copy.|
What he is saying, a single machine can run only one OS at a time, but if it is set up to, you can choose which one to use at boot up.
That said, we come to how you made the copy.... If you used a program like Acronis and made a clone of the entire drive, it should be bootable, but you will need to start the machine using the F8 option so you can choose which hard drive to boot to, but you first need to set up both hard drives as bootable and set the boot order so one is the default drive if you just turn it on normally.
If you just manually copied the files or the partition, it will not be bootable unless you at least do a start up repair on the disk with only the one drive installed (assuming you have copied all the required files and/or partition(s)).
If you used another method, please give the details.
If you used Windows Back up to make a disk image of the drive, this is not bootable, but can be restored at anytime/need to the same drive or a new one/formatted one and the result will be bootable (subject to the same conditions above if you are not just swapping one drive for the other one). This disk image will need to be for the entire drive (not just partition) or you will need to rebuild the boot partition as outlined above.
Added: another note, each copy of Windows will consider its own active partition as being on C Drive and assign different letters to all other drives that are connected. When you switch to the other copy, it will also consider its own drive as C Drive so it can get confusing. This is why you need to label each drive within each OS consistently (names) to help you keep them straight.
You have to be a little bit crazy to keep you from going insane.