2010 iMac with new firmware IM112.0057.03B (2015-002) can't install windows 7-10 anymore
I find it surprising you were able to boot the Windows installer via a USB port. I have year 2006, 2007, 20011 and 2013 iMac models and only the 2013 model can install Windows correctly using a USB flash drive installer. I not saying you did not install Windows via the USB port, rather that you probably were not suppose to. (I assume your have a mid 2010 iMac.)

When you were able to install from the USB port, I would like to know if you used a BIOS or EFI boot method. Although, knowing would not help in answering your question.

I am interest if Windows 10 was a clean install or an upgrade from Windows 7? If a clean install, then where did you acquire the "Boot Camp Support Software"? Answering might help in determining why you are having problems with Windows 10.

Below are the links to two alternative methods for installing Windows 7. Both require you to install a free copy of VirtualBox, which is only used to help install Windows 7.

"Boot camp install of Windows 7 issue, no bootable devices"

"Installing Windows 7 Pro 64 bit without DVD or Flash Drive"

Below is another example tested on Windows 10, but probably could be adapted to work with windows 7 installed on a internal disk. I have not actually tried this one, so use at your own risk.

"Is it possible to use Boot Camp with Windows 10 from an external HDD?"

If you have any questions, post a comment.