HI Anders. I'm Greg, 10 years awarded Windows MVP, specializing in Installation, Performance, Troubleshooting and Activation, here to help you.
- With both drives plugged in (if possible) please post a screenshot of Disk Management, which I read like a doctor reads X-rays. Follow the steps here so I can see everything we need to advise you: http://answers.microsoft.com/en-us/windows/foru...
- Test the hard drive and SSD with diagnostics:
https://www.lifewire.com/free-hard-drive-testin...
https://mashtips.com/ssd-health-test-and-perfor...
HP and Dell have hardware diagnostics triggered by tapping the ESC or F12 keys respectively as you power on.
Best is to use the HDD/SSD manufacturer's own if there is one, otherwise use Seatools bootable long test: http://blog.nowherelan.com/2013/04/04/boot-seat...
Follow this up with a full Disk Check from the Command Prompt in Repair Mode or from bootable media: https://www.groovypost.com/howto/check-hard-dri... if necessary from the Command Prompt at boot: https://www.tenforums.com/tutorials/2294-advanc...
- If there is no problem found with the HDD, since it has already been identified as the problem blocking boot try changing the cable, try another SATA port.
If those don't help then the problem could be old boot code interfering. Move the data off to wipe the drive with Clean Command to clear code from boot sector and partition table, then reformat and test it again:
http://www.howtogeek.com/235824/how-to-clean-a-...
Feel free to ask back any questions. Based on the results you post back I may have other suggestions if necessary.
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