You might get a bit more help in the MC21 forum since that's the current version now, as of maybe 6 or 8 weeks ago.
My answer is, I'm not sure if MC can do what you are after. If you import media into MC that lives on an external drive, MC is happy to do that. It leaves the media on the external drive and knows where it is. Whenever you want to play it, you click on it's name (or icon or whatever) and MC plays it. When you unplug the drive, MC can't see it any more. It knows where the file used to be. ...and if you set the "fix broken links" setting correctly, it will show you a red minus ( - ) next to the file name, indicating that it can no longer find the file. We're ok up to this point.
When you look at the properties of the file (or just hover over it), you'll see it's file name. Let's say your external drive was drive F:. You'll see F:\Path\To\Some\media\cool_media_file.mp4 . So you would know that you need to mount drive F, and you'd plug in drive F and everything would work properly. Here's where it gets weird for me.
If you have several drives (I'm betting you have at least 3, and maybe as many as 8 or 10), do they always get the same drive letter each time? Can you guarantee that a specific drive will show up as a specific drive letter? If so, you can easily use MC in this way and just plug in the right drive when you want to access media on that drive. MC will know the file name, which will tell you which drive it is.
I think that's probably not how Windows works with external drives though. I think it assigns drive letters dynamically and gives the first letter to the first drive it sees. I recall that Windows now has some way of mounting drives without drive letters, and using paths instead, sort of like Unix, Linux, and OS X. I don't know much about that though.
Hopefully someone more knowledgeable with Windows and external drives will step in here with some good info.
I guess the summary here is I think it's mostly possible with some specific windows techniques for managing your external drive. But I don't think there's any way for MC to *tell* you to go mount a specific drive; you'll have to look at the file properties and go mount the drive based on the path name and/or drive letter.
I hope this helps some.
Brian.