I tried all your basic things like mounting the Windows Recovery Drive, assiging a Drive letter. And following virtually all steps in this Reddit Thread which I recommend you try first, as this solves some people’s issues and is a lot less time consuming than my eventual solution.
If the steps outlined in the above Reddit thread don’t solve your problem, then you can try what I did to solve my Lenovo Z575 Windows 10 Upgrade issue.
Note, the problem generally has to do with your Master Boot Record being on a smaller secondary drive volume, and I predict this issue mostly occurs on Vendor laptops where they have system recovery volumes defined. Here is what I did.
Create a Bootable Windows PE USB Stick with Macrium Reflect Free
If you are on a Windows 7 system you are trying to upgrade, make sure you select Windows PE 3.1 in the Create Windows PE boot devices options.
After this USB stick is created, then boot off the Windows PE boot stick, select your Windows 7 installation, and make sure you select C: drive as the drive you want to be your boot drive.
Click Fix Boot Problems and make sure all the selected are checked.
Finish and reboot without the USB stick in your computer.
Now restart your Windows 10 Upgrade and it should work, because you have now shifted the BCD and the Master Boot Record to your C: drive instead of the system recovery partition, though this means you no longer have a viable system recovery option from the vendor like Lenovo, so make sure you create a recovery partition with Windows 10 after it upgrades.
If the Reddit link and steps or the steps above don’t work for you, then you may be stuck having to do a fresh new install instead of an upgrade.