The HP site shows 2 5? bays and a 3? listed as "other". If one of the 5? bays is available, HD carriers are available that will plug in like CD ROM devices. If nothing occupies the "other" bay, this should be usable. These are mostly meant for Zip and tape drives but HDs can be made to adapt. If neither of these is an option, you might be better off buying an external enclosure for the drive you've already purchased. These won't add an appreciable load to your PS which another internal will. Many name brand PCs come with little reserve for adding such devices.
I own an HP Pavilion 6913 that has only one internal HD rack/cage. Where/how can I get a second rack/cage or a dual rack/cage to install so that I may add a second internal HD -- which I've already purchased.
Thanks for any advice/suggestions -- Phil

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