Which saves

There is an implied antecedent "using plug-in drives" or "the use of plug-in drives".

I had thought of this problem, but my understanding was, using WHICH to refer to the entire thing mentioned can be used only after a comma, being non-restrictive; do you agree?

OttoJI had thought of this problem, but my understanding was, using WHICH to refer to the entire thing mentioned can be used only after a comma, being non-restrictive; do you agree?

Right, I wasn't focusing on that aspect, but, yes, I think a comma would be preferable.

No. "Which" is usually used for non-restrictive clauses but also sometimes in restrictive clauses.
"That" is only for restrictive clauses.

In addition, it is not incorrect to add a comma in your sentence, since the additional information is non-essential.

Some of the smaller notebooks use plug-in drives, which saves power and makes them even more portable.

I would always include a comma in sentences of this type (when "which" refers back to a clause).

Thank you, but the point is, when WHICH is used to refer to the previous clause, the comma must be there, at least I was told so. But according to your answer, do you mean the comma in this situation is optional and neither is better than the other? (=using a comma is no better than not using it)

OttoJThank you, but the point is, when WHICH is used to refer to the previous clause, the comma must be there, at least I was told so. But according to your answer, do you mean the comma in this situation is optional and neither is better than the other? (=using a comma is no better than not using it)

As I said, I always include it when the relative pronoun re

Thank you.

May I ask, which is your first language? UK or US?