Thats really weird that it would be doing that. Since your having so much trouble updated through the PC lets try it another way ok.
1) Go to your desktop and create a new folder called MR5
2) RIGHT click on the MightyROM5_05.08.09_Herman.exe and scroll down to EXTRACT TO, left click on EXTRACT TO. Now navigate to the MR5 folder that you created on your desktop. Extract the files from the .exe there.
3) Change the Pagepool to whatever you want using the pagepool changer in the MR5 folder if you want to. Skip this if you don't want to.
4) In the MR5 folder, RIGHT click on the RUUSigned.nbh and then select RENAME. Rename it to RAPHIMG.nbh
5) Connect your Device and allow it to activesync. Select EXPLORE on the activesync screen after it connects and synchronizes. Navigate to the root folder of your SD card.
6) Copy RAPHIMG.nbh from the MR5 folder on your desktop pc to the root directory of your SD card.
7) Disconnect your device from your PC.
Put your phone into Bootloader mode by simultaneously prssing the power button, holding the down volume button and depressing the soft reboot button. Hold until you see the bootloader screen.
9) It will immediately see the ROM image in the root directory of your SD card. It will now ask you to press the POWER button to begin the flash process. Press the POWER button now.
10) Watch the flashing magic that is Mighty Rom 5. Remember to Hard reset via clear storage AFTER the new rom image flashes and then customizes..
Let me know how you make out. You'll notice this method is actually faster than doing it through your PC and you have less troubles.