If it turns out the red dot does not indicate HDD activity, if you do a web search there is third party software you could install. If you search for Activity indicator for Windows 8.1, you should find several results.
Please let me know the outcome. I would like to know for sure if the LED also acts as the activity indicator.
Hope to hear back from you soon.
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To clarify your model is the 15-5010nr and not the 1510nr correct?
As I do not have access to lab machine of that model and the information I have been unable to locate and additional information on hard drive activity indication, I suggest contacting HP support for assistance.