All of my Arduino Uno not recognized in PC after using external power supply

Hello everybody,
Sorry about my poor english, I’ll try my best.

I am creating a Segway with arduino uno (original one).
The arduino gets power supply throw the 5v dc output in Sabertooth25X2 (the motor controller). The Sabertooth get power from 4 Lead-acid batterys (total: 24V, 18ah)
The arduino connected to MPU6050 and pass data to Sabertooth for operating the motor (Wheelchair motors)

The problem that i’ll describe happend in 2 different and complitly working boards:
When I powered the circuit with the arduino USB cable it worked good and I uploaded the code few times.
When I powered the circuit with the Battaries to Sabertooth to arduino it worked good also.
And then - the next time I tried to connect the arduino for changing the code the problem started:
The board appears in universal serial bus (usb) as “Unknown device”. nothing can change it (when i’m trying to define drivers from the arduino folder, I get a window that says driver device software is already installed (and USB symbol + write “Unkown device”).
When the board is connected the ON led + the orange led is working.
I did an experiment and switched the ATMega chip in one of the problematic boards with working board chip and the problem returned (What means the problem is in the board and not in the chip).

I tried few usb ports, few cables…nothing changed. the board seems to look not dameged and not smell burned.
When I’m connecting working arduino to the computer it’s work fine (the computer recognize it). and when I’m connecting the dameged boards it’s back.

Another clue - in one of the boards, when the problem started, the 3.3V output started to behave wired (output of about 1V…after few minutes output of 0.2V…then 0V)

I am attaching the connections that I did (one note - In one of the boards I connected the V+ from sabertooth to 5V pin, and on the next try I connected it to the Vin pin)
I am attaching the code also, if it’s going to help:
http://www.filedropper.com/mysegwaycodev3

For summary, I don’t know if I dameged the arduino in some way and what did it. It is very wired because the only different between working and non-working board is the usage in external power (that checked and gives exactly 5V). I am new in the arduino buisness…

I very need your help, thanks a lot!

Noam

If you provide 5V to the Arduino, use the 5V pin, not the Vin.

But what's powered from the Arduino? Is it just controlling some drivers? En is the 5V from the Sabertooth stable? No weird spikes?

Are you sure that the Sabertooth's 5V output is truly giving 5V? It sounds like you've possibly applied more than 5V to the Arduino's 5V pins and fried something.

Measured by AVOMeter and it was exactly 5V.

noamnri: Measured by AVOMeter and it was exactly 5V.

Well that's a good thing at least.