Hi My Name is Charley with Tech Checkers and I will be helping you with this issue:
Firstly thank-you for explaining you issue clearly.
Flash Drive have a limited number of Read/Write cycles (around 100,000) and incorrectly "unmounting" the drive can cuase the issue you are experiencing, if you are running Windows you fist step to run the Windows Utility "CheckDisk", right -click on the Drive and Select "Properties" from the Base of the Right-Click Context Menu then the 2nd along "Tools" tab at the top and "Error-checking" and "Check Now" , place a checkmark in both boxes and select "Start"
If that works , fantastic, please review here:
http://www.wikihow.com/Unmount-a-Drive
for how to correctly unmount an USB Drive
If the above is unsucessful EASUS make a FREE data recovery program for Flash Drive recovery Here:
http://www.easeus.com/resource/drive/usb-flash-drive.htm