Depends on if it's a SATA drive or not. It might also depend on how easy it is to hook up the drive & transfer the files. Yes, you can a) open your case up & install the drive in another bay, hooking up the data & power cables, b) make sure that the new drive isn't 1st in the boot order in your BIOS, c) copy the files you need, d) wipe the drive, & e) repeat steps b & a in reverse order; or, you can forego the bay installation, just leaving your case open for the few hours (or days) that it might take to transfer the files; or he can use the transfer device & not have to reboot his PC or even open his case.
The latter was extremely nice the last time. I kept having issues transferring data from the WD Black to the secondary WD Green (was using Linux Mint DVD as a temporary OS), where it would freeze up after maybe 20-30 minutes. After buying new drives, I realized that the problem was the data port on the Green -- the plastic tab had broken off a few years back, & was stuck in the data cable. It was much quicker to simply unplug & reconnect the USB cable on the external adapter than to reboot my PC every time.