If you're saying it can be installed in
a desktop, not all external drives can.
Western Digital has quite a few drives that has the USB controller mounted directly to the drive. There is no SATA interface on it, at all. So while yes, you can install it in the machine, you would still be limited to using it as a USB drive, not SATA. Here [mpecsinc.ca]
is an example.
That was an exception rather than the rule. I believe WD had a few drives with a form factor that was different from the typical 2.5 or 3.5 with these built in connectors. But you are correct, not ALL drives can be pulled out, but ALMOST ALL can be and that is easy enough to verify with a web search.
Golden Rule of a Deal: If an item has been offered validly at a price of $X at any time, then never pay more than that in the future. It has been discontinued or will appear again at that price soon enough.