I went to boot 10 from my secondary hard drive and it would no longer boot. I tested the drive and it passed the SMART tests and a simple read test but failed on a quick self test. Trying to repair the Win 10 installation failed so I wiped the drive and removed it. It is an older 2007 model Western Digital 500gb that I bought used. It seemed to still work fine for copying things and storing backups but I didn't trust it and ordered a new Seagate 500gb drive for $38 to replace it. It also was very warm when I took it out. Much warmer than the 1tb Seagate Barracuda that is the primary drive. I could always hear it running while it was booting before but when it was trying to boot yesterday it sounded like it was getting stuck.
I will not have Win 10 now until I get the free upgrade in about 3 weeks. I'm not going to bother with installing a preview build on the new drive. It will be used strictly for storage and image backups. I think I've seen what I need to see of 10.