Hallo
When i Try to start Monitor Mode with Airodump-ng in backbox i got this error message !
my wifi usb has worked under backtrack 5 with airodump-ng !
i have tryed  it with all my usb who works under bt5 etc. to start montor mode in backbox but  without sucess!  

er74b1@er74b1-ASE571-AST671:~$ sudo airmon-ng [sudo] password for er74b1: Interface   Chipset      Driver wlan0      Unknown       r8712u wlan1      Unknown       usb er74b1@er74b1-ASE571-AST671:~$ sudo airmon-ng start wlan1 Found 5 processes that could cause trouble. If airodump-ng, aireplay-ng or airtun-ng stops working after a short period of time, you may want to kill (some of) them! PID   Name 582   avahi-daemon 583   avahi-daemon 584   NetworkManager 623   wpa_supplicant 1536   dhclient Process with PID 1536 (dhclient) is running on interface wlan0 Interface   Chipset      Driver wlan0      Unknown       r8712u wlan1      Unknown       usb (monitor mode enabled) er74b1@er74b1-ASE571-AST671:~$ sudo airodump-ng mon0 Interface mon0: ioctl(SIOCGIFINDEX) failed: No such device er74b1@er74b1-ASE571-AST671:~$

Thanks! this  Start the airodump-ng windows but does monitor 

lo        no wireless extensions.

eth0      no wireless extensions.

wlan0     IEEE 802.11bg  ESSID:"JOHNDOE"  Nickname:"rtl_wifi"
          Mode:Managed  Frequency:2.437 GHz  Access Point: 00:01:E3:C5:78:4F   
          Bit Rate:54 Mb/s   Sensitivity:0/0 
          Retry:off   RTS thr:off   Fragment thr:off
          Power Management:off
          Link Quality=95/100  Signal level=48/100  Noise level=0/100
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

wlan1     Ralink STA  ESSID:""  Nickname:"RT2870STA"
          Mode:Auto  Frequency=2.412 GHz  Access Point: Not-Associated   
          Bit Rate:1 Mb/s   
          RTS thr:off   Fragment thr:off
          Link Quality=10/100  Signal level:0 dBm  Noise level:-115 dBm
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

Does work !