COM Port problem with MEGA2560 and Windows 7 Ultimate (32)

Hi everyone,

I'm using an arduino MEGA2560 and a Data acquisition system, that is basically an arduino LEONARDO with an accelerometer and an analogic sensor on Windows 7 Ultimate (32).
The sketch of the MEGA is very simple and works just fine, the idea is to send a periodic signal to LEONARDO (ON/OFF pin), and the led is used as a visual control of this.

The LEONARDO has its own software that plots the data from the accelerometer and the sensor in real time. It shows that everything is working fine (the sensor read the ON/OFF state of the pin).

When I try to load the sketch of my project on the MEGA I get:

the selected serial port
does not exist or your board is not connected
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_getsync(): timeout communicating with programmer

But, the board is connected (in fact it's blinking!). This happens with any sketch, and unplugging the LEONARDO. The only way I can take back the control of the MEGA is rebooting mi computer.
The MEGA always is on COM5, and the LEONARDO on COM10, and both are detected by the IDE and the device manager of Windows.

I search for a solution but don't find a case where the port is actually detected but not working.
I tried all the suggestions in except "Try uploading with nothing connected to the board (apart from the USB cable, of course).", because is a big headache I'm trying to avoid because the project involves another arduino (MEGA1280) as slave that controls a servo, a sensor force and a shield that plays a bip using tone library.

Maybe I'm not searching with the rigth words, but I'm from Argentina so I'm doing my best effort to make me understand.

Any help appreciated. Thanks!