I had trouble updating my maps too, first attempt stopped part way through and the car filled with "uh-oh" sounds, tried again same thing happened. Tried again next day, no luck. Then wiped the usb drive and tried again, failed again. Wiped another usb drive and tried installing from that and it worked!
Be aware that when the screen says it will take about 30 minutes, it won't, it's more like 1 1/4 hours, so you need to be patient and have the engine running whilst it's installing.
How I did it...(do at your own risk, no liability will be attributed to myself for following any of the instructions below)
1 Download License and map files.
2 Format 8GB+ USB flash drive (backup anything you want to keep from it first)
3 Unzip the map files to your computer's hard disk.
4 Copy the license file to the USB drive
5 Copy the four unzipped map files to the USB drive
6 Switch on car engine.
7 Plug USB drive into the car and follow the on screen instructions.
8 If it fails try another USB drive and repeat steps 2,4,5,6,7 again.
9 Wait/drive around for 1 to 1 1/2 hours, don't switch engine or radio off!