With pc switched off and unplugged from mains (obviously you already know that:)) just disconnect the eide drive. You will no longer have access to the files you have stored on there.
Not sure what you mean by "I thought, erroneously, that it would add capacity but it has just been running in parallel with the 160Gb now nbr one C Drive."
It has added to your storage capacity - 40Gb's worth.
I presume the 40Gb drive has it's own drive letter.
Only way it would slow things down is the speed of the drives but I would have thought that minimal and hardly noticeable.