What is my responsibility to my lab after I graduate?
Here's a way of approaching the ethical aspect of this: how important would it be for you, if you were in the middle of your degree, that a recent grad's files were left in a big mess?  Would that affect the finances of the group, because of the files eating up a lot of storage space?  Would your work be more difficult or time consuming, with needed files being hard to find?  Would you have to take some time away from studies and research in order to try to put the recent grad's files in order, for the good of the group?  Would it be difficult for another member of the group to figure out what was what, in the recent grad's absence?

I wonder if you could ask for payment for at least part of the extra time you'd be putting in.