"I want to leave all the data on the external hard drive in case this computer crashes too."

What if the external hard drive crashes?

You still need a two copies.  I would just use the C drive as usual, and keep using the external hard drive (and/or a cloud backup system) as a back up.

If your current computer is "having issues", then I would recommend either contacting an IT expert and/or start shopping for a new computer.