Advice on "unbricking" a soft-bricked stick

I've been scouring different forums for nearly a month now and can't find a solution that works for my situation. I resisted posting here, hoping to find the info on my own without any luck. I have a handful (6) of these generic MK809III Rockchip 3188 based sticks. All identical. I have no idea which Chinese manufacturer assembled them. Four of them work perfectly. Two of them have been corrupted by me. I inadvertently wiped out the build.prop files. The files are there, but they are empty. (I know this because the last one I botched, I could see the file in the open editor with no content, as a read-only. The moment I closed the screen... that was it.

Using the "hold the pin in hole, then power up, wait 5 or 10 seconds, then release the pin" method of recovery (sorry, I don't know the proper name) does boot it into recovery mode, but the screen shows this:
Failed to mount /mnt/external_sd (No such device)
Failed to mount /mnt/external_sd (No such device)
Failed to mount /mnt/external_sd (No such device)
one_data_if_exist->can't find databk partition
Formatting /data...
Formatting /cache...
Booting...
--- then, a stuck boot animation.

My problem, is that I cannot connect to these via USB to do anything to them. My system tools are:

Linux 14.04 LTS workstation
RK_Flashtool_1.7
Android Studio
Latest Android tools sdk
Other tools available to me: A Mac (running Yosemite OSx)

Notes:
- Adb drivers, I assume, are correct, because I can access the working stick computer via ADB
- The RKFlashtool will not recognize any of the corrupted units, or show that it is even connected
- Obviously, I cannot set the "Connect to PC" flag without the OS booted
- In a terminal window, adb does not show the usb device as being connected
- I do not have a back up for any of these. I tried, but both Clockwork and TWRP would not recognize the unit to permit a backup. The unit is recognized as: "get_dongle_3188".

I really need some advice. I must be overlooking something, I admit to be very new to Android. I wish it were possible to backup one of the working units, and install it on the corrupt units. But no backup app was installed in the boot loader of the corrupted (that I'm aware of).

Thanks for your time!