The "D" partition is for HP Recovery media. It should not be used for any other purpose. It appears that a program/s is saving data to it arbitrarily. You can carefully move any of the information that is not related to HP Recovery. Then, make certain when you save data that it is not being saved to the "D" partition.
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Intelligence is God given. Wisdom is the sum of our mistakes!!
I do not know that I used D for any purpose. I can find no more data on it at all. I tried all I could before resorting to the forum. Although it shows as full for all practical purposes in explorer, opening it shows nothing at all in it. Insofar as I know, I am not saving data to the D partition.