external hard drive not found in mavericks. help

Hi

I have a macbook air (mid 2012) running the latest version of OS X Mavericks. I had to repair my flash storage in an Apple Retail Store to get MAvericks installed. Ever since I've had Mavericks, my two 1TB usb 3 external hard drives (one is samsung, one is toshiba) are not showing up in Finder or on the Desktop.

They showed up and worked fine in Mountain Lion but they haven't worked ever since the upgrade. I've verifyed and tried to mount the disks in disk utility but it says they can't be mounted. Anybody else had this problem and how to fix it?

I can't really repair or rease them because they are full and I don't have a backup for them.