Q: Bootcamp problem: Yosemite 10.10.5, Windows 8.1

Please see Create an ISO image for Boot Camp from Windows installation media - Apple Support, but you also need a functional optical drive internal or external.

Is this a single DVD with 32-bit and 64-bit on it? If so, such a DVD does not work, because BCA cannot tell which version you want to install.

Please boot in Safe Mode - Try safe mode if your Mac doesn't finish starting up - Apple Support - and then boot normally and try BCA. If that does not work, please try Using Disk Utility to verify or repair disks - Apple Support .

Can you run BCA and just click on option to Download Software? It will ask you for your USB which will be formatted and erased.

Also can you check /Applications -> Utilities -> Console logs  for any error messages related for disks? Are you using any hubs?

Run Bootcamp Assistant and click on the second check box only and  click continue.

Was the download successful?

Excellent. Can you check /Library/Application\ Support/Bootcamp for a file called WindowsSupport.dmg? Please post the lines similar to the following from Bootcamp.xml on your USB drive for the record.

<ProductManufacturer>Apple Inc.</ProductManufacturer>

  <ProductVersion>6.0.6133</ProductVersion>

<ProductCode>{FCFFE6B6-BAE8-490E-88D0-097A9DA1C43D}</ProductCode>

If it is still there, build the full installer (current USB only has the BC drivers) by clicking all three options and then install Windows.