I had a similar problem with flashing a custom recovery on the tf700 as well (mainly because of the dock), so let me share with you a few pointers that may help you fix this (it certainly helped in my case):
1. Never do any flashing, adb or fastboot with the tablet docked, especially when flashing recovery: I spent three hours scratching my head figuring out why I couldn’t flash the recovery (fastboot stuck at sending file), only to find out that everything worked perfectly when I removed the dock. In my opinion, the dock makes things unpredictable, so do everything with the dock.
2. As for adb and fastboot, you should just start setting up from scratch as it is always easier than troubleshooting for errors. After unlocking the bootloader using ASUS tool, what I suggest you should do are as below:
a. Uninstall COMPLETELY your current adb/fastboot drivers and install Universal Naked Driver 0.73 (http://forum.xda-developers.com/show....php?t=2513339
). Take note of the warning in red on how to uninstall existing adb driver.
b. Also take note that you actually need 2 sets of drivers, 1 for adb and 1 for fastboot (which are both included in the Universal Naked Driver). When you connect your tablet with Android running to your PC, you are installing driver for adb. With the driver properly installed, “adb devices” command will work and give you the list of connected devices to your PC through Windows’ CMD. Please also Google or search xda for adb files (no need to download the whole Android SDK just to get those adb files).
c. After successfully installing adb driver, you can manually reboot your tablet into fastboot mode through the button combo (power + volume down) with the tablet still connected to your PC. By then, your Device Manager should have another device listed as fastboot device. Again, follow the steps in the link to uninstall and reinstall fastboot driver. With this set up, you should be able to run “fastboot devices” command.
d. With fastboot, you can now flash a custom recovery. As a side note, since all this flashing custom recovery and ROM can potentially brick your tablet, after you have finished fixing adb/fastboot and get your ROM running, I strongly suggest you follow this link to obtain nvflash (http://forum.xda-developers.com/show....php?t=2455925
) and make your tablet unbrickable. I actually did this before flashing a custom ROM, but I honestly don’t know if the timing makes any difference.