USB Thumb Drive Does Not Appear in Finder or Disk Utility

I have a new Macbook Pro with Retina Display (early 2015) running OS X Yosemite (10.10.3). I have tried to put use different USB thumb drives and neither appear anywhere on the computer (including the Finder, System Report, and Disk Utility). When I remove them from the port I do not receive a warning about having to eject the devices first. The thumb drives should be good as I've used them on a different machine very recently. Any thoughts on how to get my Mac to recognize these drives? Thanks.