Hi Oleg,
ok, I'll try to be more clear.
So every time the mcu sends data, the PC has to reply with either READY, if no error occurs or ERROR in case of errors. If READY is received by the mcu, the transfer goes on. When it's time to end the transfer the mcu will send the FT command. If the PC received it without error, it sends the READY command. Then it's the tricky part, if the mcu receive instead of READY, something else because of an error, it sends the ERROR message and the PC will repeat the READY command. But if no error occurs, and the READY is received correctly by the mcu, it won't send anything back, and the PC will hang...

I know that it might be hard to understand, anyway hope that it's more clear now.
Regards Gilles