Lexar dual slot usb 3 card reader not working

I have been using a Lexar dual slot reader on my imac running yosemite for a few months with no problem.Suddenly its stops working (not reading cards or reader,no blue flashing led).usb ports working fine for other external drives.And if i connect same card reader to my Macbook air it reads no problem.Lexar support recommended I update the firmware but the update software "cant find device".Any help would be great...Just a small observation..I've noticed the tiny blue sparks you sometimes get when plugging in a usb have disappeared ..27 inch imac late 2013 running Yosemite