Most vendors (including Apple) won’t say which drive is used (probably a lower performing one in many cases!), but OWC spells it out. This might not matter to most users, but I want a quality case, and I want to know which drive is in it (several reasons why, out of scope here).
Kudos to OWC for making this information available. Just click on the Spec tab on the page for an external drive.
By the way, I highly recommend the 4TB Mercury Elite Pro as an external backup drive, especially for using cloning as a backup strategy. The other OWC Mercury Elite Pro quad interface models are also excellent choices, though I prefer 4TB (or 3TB) models.
OWC is an advertiser on this site— because I like and use their products, and I value their service and honesty, this is one example. I don’t take on just any advertiser.