External Hard drives

It's normally recommended to have a HD for audio / the project folders. There's no reason it should be external.

It ain't that difficult when you think about it, surely nothing strange, just simple logic.

If you only have one hard drive it has a whole lot to do, especially when recording.
It has to read the material to be monitored, write what's being  recorded, and in case of VSTs using audio samples, it has to pick and read the right samples on the fly.

The projects can contain a huge amount of audio, and even a large HD can get full faster than you expect. Note, that also the tracks you have deleted from the project stay in the audio folder unless you get rid of them. It's quite a nuisance starting to make room for new projects if your OS, videos and whatever are all on the same drive.

Having two or three HDs makes your system more reliable. Of course the content and structure of your projects plays a role. This is just a general recommendation.

Not knowing about your projects and VSTs it's hard to say, but my guess is the playback clicks are just a driver/buffer thing.