No device found on COM5 with Bossac on Winwdows 7

Palliser, thanks very much for your help !

I erased and reset your my board and I relaunch my commad line.
The messages displayed are :

Erase flash
Write 9748 bytes to flash
[==============================] 100% (39/39 pages)
Verify 9748 bytes of flash
[==============================] 100% (39/39 pages)
Verify successful
Set boot flash true
CPU reset.

The program uploaded is running correctly ! Yes !

But if I try to relaunch my command line, It doesn't work !!!
I must erase and reset my board before upload my program...

But, I don't understand why the message displayed is different of Arduino IDE:

Erase flash
Write 9748 bytes to flash

[                              ] 0% (0/39 pages)
[=======                       ] 25% (10/39 pages)
[===============               ] 51% (20/39 pages)
[=======================       ] 76% (30/39 pages)
[==============================] 100% (39/39 pages)
Verify 9748 bytes of flash

[                              ] 0% (0/39 pages)
[=======                       ] 25% (10/39 pages)
[===============               ] 51% (20/39 pages)
[=======================       ] 76% (30/39 pages)
[==============================] 100% (39/39 pages)
Verify successful
Set boot flash true
CPU reset.

And why I must erase and reset my board each time I want to upload a program ?

Thanks.

Nassa.