My sys crashed, since then I get USB device not recognised error.Only for devices that the sys sees as a drive?

Yep it happens to many People uninstall the device from the device manager and do not reuse it till you have read the info from Mcrosoft regarding your system, specially old laptops get the Blue screen of death caused by a USB_BUG_ERROR at 0x000000123 etc., you must find the answer fix it you may need a Bios upgrade a Windows patch or a better driver. You'll find more info in your system file log in the Computer management Files.