How to keep track of external hard drives?
This is an interesting (as in unique, but not something I would personally do) way to store files.

My first thought would be to use colored tags.


Put a colored label on each of your external drives
Tag the aliased file/folder with the color associated with the drive's colored label.


Below, I created a folder alias (Music) from a USB drive to my desktop then  I tagged the folder with the red tag to correspond with the color coded USB drive.  





The colored tag on the folder alias will tell you which drive the alias resolves to.

You can even use multiple colors on the tag if your drives go above 7.  Here I have blue, orange, and red tagged.



You can find more information on how to fully use Tags from the Apple Support Document:  OS X: Tags help you organize your files