I have a 3G dongle plugged into my Pi. Running lsusb only occasionally shows the device occasionally in the list. When I tail /var/log/messages I see the following:
Feb 7 17:09:07 raspberrypi kernel: [ 253.960592] usb 1-1.2: USB disconnect, device number 40
Feb 7 17:09:11 raspberrypi kernel: [ 258.290719] usb 1-1.2: new high-speed USB device number 41 using dwc_otg
Feb 7 17:09:11 raspberrypi kernel: [ 258.393204] usb 1-1.2: New USB device found, idVendor=12d1, idProduct=1446
Feb 7 17:09:11 raspberrypi kernel: [ 258.393234] usb 1-1.2: New USB device strings: Mfr=3, Product=2, SerialNumber=0
Feb 7 17:09:11 raspberrypi kernel: [ 258.393251] usb 1-1.2: Product: HUAWEI Mobile
Feb 7 17:09:11 raspberrypi kernel: [ 258.393263] usb 1-1.2: Manufacturer: HUAWEI Technology
Feb 7 17:09:11 raspberrypi kernel: [ 258.399070] scsi44 : usb-storage 1-1.2:1.0
Feb 7 17:09:11 raspberrypi kernel: [ 258.402217] scsi45 : usb-storage 1-1.2:1.1
Feb 7 17:09:11 raspberrypi kernel: [ 258.825137] usb 1-1.2: USB disconnect, device number 41
Feb 7 17:09:20 raspberrypi kernel: [ 267.250707] usb 1-1.2: new high-speed USB device number 42 using dwc_otg
Feb 7 17:09:20 raspberrypi kernel: [ 267.353446] usb 1-1.2: New USB device found, idVendor=12d1, idProduct=1446
Feb 7 17:09:20 raspberrypi kernel: [ 267.353477] usb 1-1.2: New USB device strings: Mfr=3, Product=2, SerialNumber=0
Feb 7 17:09:20 raspberrypi kernel: [ 267.353494] usb 1-1.2: Product: HUAWEI Mobile
Feb 7 17:09:20 raspberrypi kernel: [ 267.353506] usb 1-1.2: Manufacturer: HUAWEI Technology
Is it likely this is due to a mounting / software issue or could it be power related?