Please check the post how to change the default of the CP2102 USB to serial chips from bus powered to self powered.
This stops the risk of frying the chip if a laptop continues running when the mains power to the XMs is switched off or cut.
New generators should now be OK, but the previous 2 years XMs should all be changed. The program is used once, and changes the Bm Attributes from 80 to C0, this can be checked using my favourite free USBview.exe, on here or from Microsoft. I have a Silabs utility which will change the attributes, but only one at a time.
We always recommend using a powered USB hub. I have found my old XMs will send 1.6 volts to the USB when they are connected to the mains but switched off.
For more details, please check the link: