@ Peter: My NIDAQmx version is 9.5.1 f1 and not 8.5.1. Sorry for this mistake.
You wrote : 1- Check if the "NI Device Loader" service is active. I thought it was, because it appeared in the bar and is launched at restart.
The link you added is not ok for me and Firefox said "Firefox can't find server at ae.natinst.com." I've found this one "http://digital.ni.com/public.nsf/allkb/0819391CA91B3EB38625731D00024649?OpenDocument" which seems to be the same. I've now understood what you meant by "Make the settings on the recovery-tab of the NI Device Loader Service" This is the real key of the problem and it works better when I reboot the machine.
If NI MAX fails I know what to do in a few seconds! Great!
External USB hubs are not officially supported by NI or any of NIs USB devices, however, they often work with no problems. That said if you're going to use one I would strongly recommend using a powered one, I wouldn't be surprised if an unpowered one gave you problems.
Mireka and Ajeetraj, thanks for your posts, I think your two problems are different and unrelated, Ajeetraj's device disconnects while running, but Mirekal's device will continue running fine once it's connected. Mirekal; please bring your problem up in another thread and make sure you mention that you've already tried the solution from this KB as this is the first thing people will suggest.
Ajeetraj, what PC are you using specifically, and which chipsets does it have?
Joel G Applications Engineer National Instruments UK & Ireland