Recently, when I boot my machine, Windows 10 would get stuck on a "Preparing for startup repair" screen. This screen would stay there forever, with the "spinning loading dots" animation.
After 5 to 6 hard resets of my machine Windows would boot up quickly and properly with no errors. Some daysit would boot on the first try, and sometimes it would take up to 10 reboots.
I tried to reinstall Windows 10 from a USB stick, but even the USB stick would be stuck on a "never-ending startup" until after 4 or 5 reboots, at which point the Windows 10 installer ran properly and without any issues.
Now, after a brand new Windows 10 install on my formatted SSD, I am having the same issue – Windows still refuses to start properly, instead going into "startup repair preparation" about every 5 out of 6 times.
I was tempted to say it is a problem with the SSD, but it is strange that my machine had the same trouble booting off of the USB stick as well.
I scanned the main Windows partition on the SSD with the "right click->scan drive for errors" feature in Windows and it found no errors.
Best Answer
I would suggest this may be a boot virus. Reloading Windows may not rewrite the boot sector, if it's repairing an existing Win installation. It may not even rewrite the boot sector if you've reinstalled on the same partition, meaning you didn't repartition the drive before the new install.
https://neosmart.net/wiki/fix-mbr/
That page has instructions on how to rewrite the boot sector on several Win OSes, but here's a snippet for Win10:
There's a 2nd part to this fix, but that's only if this doesn't work and you know it's the boot sector giving you the problem. I'm not including that here, since it's pretty long, and hopefully Part 1 works.
This is at least the First thing I'd do, given what you've already done.
Since this is happening on your USB stick, you may want to do this on that drive, too.