I have an Edimax EW-7811UN WiFi dongle which I bought for a raspberry pi. I have not managed to get it to work so am testing on my laptop for easier fault finding and since I know the installation and wireless (with built in adaptor) is working fine.
If I disable the built in adaptor (wlan0) and enable and configure the USB Edimax adaptor (wlan2), it constantly tries to connect. Then it will sometimes manage but then immediately drop the connection and try again.
There is no MAC filtering on the router. Any other device works without any problems. I am using WPA2, if that makes a difference.
I have trawled the net and tried every fix of alternative drivers that I could find.. but still no luck. But most of these are for if it isn't recognised.