On my 6 year old HP desktop there's a key I can press that will allow me to tell the computer which drive to boot from. Look at what comes up when you first boot. I have to wonder though why the 2nd drive was installed if there was nothing wrong with the first one. Are you sure that drive still contains an intact bootable version of Windows?
My
HP Pavillion
Serial: 4CE1381B2R
Product: QU139AA#ABA
Model: p7-1108p
turns on as though the hard drive died. Fan sound is there, but no output. A second drive was installed a couple of years ago and I would like to switch to the original. Both are connected. Do I simply disconnect tne new drive?

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