First of all make sure that you don't have the write-lock or write-protect switch enabled on the SDCard or USB drive.
If the above is not the case, then follow these instructions:
Hit "Windows Key+R" and enter "cmd" to enter the command prompt:
Now type the following :
After each step hit the enter key
type LIST VOLUME
type SELECT VOLUME X (where X is the volume number you want to remove the write protect from.)
type ATTRIBUTES DISK CLEAR READONLY