I've had a 4424 for a while now and recently it has been 'dropping off' for no apparent reason. The software will popup the 'Check USB cable...' dialog box and it usually hangs. I can upplug the USB cable from the PC and plug it back in and about 10% of the time, it will then work fine, most of the time it does not. The red light is on in front of the scope, but the s/w does not find it. If I plug it in a different port and try several times, it will eventually be recognized but then during operation it will eventually get 'lost' again. I have observed this behavior on several computers, so it does not seem to be a PC-specific issue.