[solved]Cant upload to nano from PC. Ok to Mega,Uno -Ok to same nano from laptop

hello, i'm experiencing the following problem:
I have 5 nano clones.

From time to time from a specific pc (win 7, ide 1.8.1) I cannot upload any sketch to them (avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x34....), not even "blink", but can upload to uno or mega (same cables, ports ...) . (so I conclude driver ch340 is ok).
On the other hand, if I use my laptop (win7, same cables, ide 1.8.1) everything is fine.

Installed 1.8.1 from the beginning, verified driver intalaltion, ->same problem

Suggestions?

Do you have anything connected to them when you upload any sketch?

Plug out everything such as Led, motors, etc.

Also, other causes are

Uploading to wrong COM PORT
a wire is connected to rx or tx pin
Drivers not installed

These types of errors usually occurs on Windows 7 computers

OMG!
that was really good. it seems that there were some remains of old installation (1.6.something). The boards.txt on that installation had been changed so to use "optiboot" on nano in order to bypass the wdt issues.
Depending on how the ide was activated (arduino.exe or assigned .ino), different parameter files were used.

the key indications on "seeing" the problem :

  1. the "bypass baud rate" message on begin of upload was 115200 (on last intallation's boards.txt was 57600), while on the laptop (working upload) was also 57600.
  2. At the end I installed ide 1.8.5. On first run I was astonished seiing "nano" and "COM3" (which were last used) as preselected board and port (where these information arrive from?).
  3. wide Configuration file in ...(different) folder

now, let's go back to work