USB Thumb Drive won't appear or mount on Desktop

I'm preparing to upgrade to Mavericks, and before I do, I want to make a bootable drive on a PNY USP 2.0 FD Media thumb drive. When I insert the thumb drive into a USB port, it doesn't mount. It doesn't appear on the desktop. The drive is listed in Disk Utility, but Verify, Mount, and other First Aid options are greyed out. I can only unmount the drive from Disk Utilities. I've checked the Finder preferences, and External Disks is checked under "Show these items on the desktop". I'm not sure if I've used this thumb drive before or not. I was trying to check this disk before I created a bootable disk. I might have already done this shortly after I purchased my iMac, but I'm not sure. Other USB drives mount fine, and show all of their contents. I'm using an iMac currently running OS X 10.8.5. I'd appreciate any hints on what might be going on here. Thanks!