1.6.10 - Can't Burn Bootloader

1.6.10 trying to burn boot loader with USBasp and I get a bunch of errors "Avrdude cannot find file..." Tried three PCs all the same five lines of errors each time.

It was only a few weeks back I converted all my Nano to Uno boot loaders with this same USBasp programmer so wondered if I was using 1.6.9 back then. Went back to that and all good. Must be a "feature" of 1.6.10. :)

Just saying in case anyone else has the problem.

Yes there are multiple issues with the new version of avrdude that was included with Arduino IDE 1.6.10. From the minimal amount of information you've provided I can't say which if any were the cause of your issue. Note that the same avrdude version is included with Arduino AVR Boards 1.6.12 so if you update to that version with any IDE version you will experience the same problems. Because of these issues the Arduino developers are reverting to the previous avrdude version on the next IDE/Arduino AVR Boards release and will only return to the new avrdude version once they have fixed all the issues.