I have a similar problem. Very similar, which is why I'm not starting a new thread. After upgrading my early 2011 13inch MBP (major difference between my and the op's problem, as I have nothing to plug into the usb ports; my keyboard, monitor, and trackpad are permanent) to Yosemite, my Windows 7 stopped booting. It'd worked without any problem whatsoever for almost three years.
My computer is early 2011 13 inch MBP, 1tb toshiba hdd, 8gb ram. I don't know if that info is relevant, but there it is.
I was traveling for work at the time, and accessing the internet on a hotel network, and when everything corrupted, I panicked, thinking that the Windows side picked up a virus or something. I nuked the whole thing. Full-on no recoverable data possible zeroed it out. I didn't think there'd be a problem, and wasn't worried about data loss since I didn't have anything but some old photos that I hadn't moved into google drive yet. I barely remember my college days anyway. A few photos won't help that.
Using my phone, I found out that many people had problems with Yosemite, and my long in the tooth laptop would have problems running it anyway. Fine; I'll just stop upgrading at Mavericks.
When I got home, I got out my old Snow Leopard install disk, reinstalled mac os, upgraded to Mavericks, updated all of my software, and started installing my copy of Windows 7 home premium. This is where it gets weird: intead of using the blank cd's that I used 3 years ago for the driver installation, I was required to use a usb drive. Didn't even have the option for cd. So I went out and bought a usb drive, downloaded the bootcamp5.1.5621 zip file, extracted it and put it on the drive.
The installation went fine. Boom, there's windows. Click on the usb setup icon. Boom, there's all the stuff. I can use Windows 7 no problem. Surf the internet. Install the game I want to play, etc. Everything works until I go back over to mac os, or shut the computer down.
After setting up windows and then heading over to mac to do some work, or if i shut down the computer, I cannot get back into windows. I get the no bootable device nonsense.
If I try holding down option, or command r, I can see Windows there. If I click on it, I get the black screen with a blinking cursor.
I've tried five times. Removing windows, reinstalling it. Same story every time. I haven't payed attention to what usb port I put the thumb drive into, so I'll try again, using the one closest to the back.
Driving me bananas.
