I have been using the arduino for a while, I know how to avoid the issues you mentioned about, like Port, USB, blabla…
In fact, this issue is more problematic than you think, on the arduino forum there are many posts about issue of uploading programs to mega 2560, none of them really solves my problem. such as arduino.cc/forum/index.php/topic … #msg493290
So I do really need some more serious technique support from you, I guess there are something wrong on the bootloader or something similar, can you check this again,
thanks