I have a Samsung M3 1TB USB 3.0 external hard drive and an internal old 500gb drive. The internal drive is in my Macbook Pro. Would it be possible to swap the external hard drive with the internal one, while keeping the internal drive usable in the external one's case?
Judging from how Samsung M3 drives look on the inside, the answer is no - it is not possible.
The reason for this is, that harddisk inside Samsung M3 doesn't ship with standard SATA controller and instead appears to have an USB3-only controller hard-wired.
Laptops generally don't have internal USB3 adapter inside their harddisk slots (it wouldn't even make sense, since SATA is faster), so i'm afraid the disks cannot be interchanged.