External hard drives are basically 2.5" (if portable) or 3.5" normal internal hard drives in an external enclosure. There are often videos on youtube and other similar sites, where you can see someone doing it (taking the drive out of the enclosure). But it's not recommended that you do it unless it is a last resort and you desperately need the data from the drive and that there's a chance, that this would help you get it, because taking a drive out of its external enclosure will most likely void the warranty and if that particular drive has hardware encryption - you won't be able to get your data, because it would be encrypted.
On the other hand, the user above is talking about an external enclosure which is sold separately e.g. you buy a normal 2.5" internal drive and a 2.5" external enclosure and put the drive in the enclosure to use it as an external HDD. In this situation you won't void the warranty of the drive if you take it out of the enclosure. In these cases, there should be instructions with the external enclosure on how to connect and remove a drive from it.
I hope I've answered your question. Let me know if there's anything else