Sysex upgrade requires only midi in on your P6. That means your computer will not know if P6 has received or not all sysex information. For this reason the MIDI must be sent at a lower pace to allow bigger sysex files to be received.

The best way to verify that all sysex is being sent, is to run a dry upgrade on all files you plan to send out to you P6, and see all finished. A dry upgrade means you do not connect the MIDI cable to your P6 MIDI IN.

If the process will finish, it means you should be good to send out your Sysex files and complete the real upgrade with the MIDI cable connected.

If dry upgrade doesn’t complete it means your MIDI app has a buffer configuration problems which would most likely kill your P6 firmware.

I sent earlier a recommendation for configuring the buffers for Midi-Ox. Check the forum for it.

Good luck!