The WD Easystore (which I think is the same as WD Elements beloved by drive shuckers) isn't a MyBook. But they are a good, simple USB enclosure with no limiting, enforced encryption, unlike the MyBooks, and you get a well priced WD drive too. I've tried to juggle good drives between MyBook enclosures, even with password security disabled the data is written scrambled based on the enclosure's interface board. So if you just want a well priced equivalent of buying separate enclosure and drive then I've been really happy (so far) with the Easystore/Elements.
I've got a 12TB WD Elements which had a white label drive. Prior to this I've had 2TB to 6TB Elements with WD Green (when they existed) to Blue inside. Never actually put them in the NAS but people do. I mostly use them for Hyper Backup destinations as USB shared folders on the NAS.
I used to use an external drive on the
DS218+ to hold a video + music library (not Photo Station/Moments as they have specific folders). The main pain was if Video Station ran and the drive wasn't mounted... VS would forget all the library, which is why I used to export .vsmeta files very regularly. Plex was a lot more forgiving as it has a time-out if not found before it cleans up missing items.
I also use WD Elements on the Mac Mini to hold its 'iTunes' Media (6TB) and a 4TB data/dump. The iTunes disk is organised so that it can work with Mac Music/TV apps but is well structured so its backup (using CCC, with safety net feature) to the NAS is ready to be used with Plex, Media Server, Audio Station, VS (as a Plex backup), Photos etc.