If you open up the housing of most 2.5 inch external HDDs, (superfluous: those that don't need and extra power source) the disk inside can be used as an internal HDD, provided it has a SATA connection. One removes the SATA-to-micro-usb-3.0-connector, et voila! It seems that some HDD's (by Toshiba and Verbatim for example) have a micro USB 3.0 card, so those can NOT be used inside laptops or network devices.
Question: How can we know before opening or better yet, before buying whether such an external portable HDD has a SATA connection and nót a micro USB 3.0 card? So, what specific characteristic in the specification does one look at? Interface?
Follow-up: What other issues might prevent us from shucking the HDD's?