Have you ever bough an external hardrive only to take the disk out of it?

Hiya, so am looking to buy more storage and while browsing am seeing some external harddisks, such as Western Digital My Book and Seagate Expansion Desktop for cheaper than the internal harddisks themselves. Have seen this one video from KTZ Systems where he bought up multiple of these external ones just to open them up and use the disks for his own server. Was therefore wondering if you peeps have ever done this and if there any downsides to it at all?