Hi Jake
This is Tamal trying to answer your question.
The native image folder contains processor specific codes for faster access of resources on the PC. The folder usually resides in the c:\windows\assembly
but in your case that is on E:, if you remember installing windows on E drive that can give a clue. The folder should not contain files more than 800-900 MB. If your PC can run fine with the external drive disconnected, then you can safely delete them. Probably the folder grew due to accumulation of data by mistake if it was used in the same computer for long as system drive and there is no explanation why it grew if it was used in different systems as portable drive only.