It may be a Permissions thing so check first in My Computer, right-click the USB Drive and select Properties>Security tab Select the user and click the Edit button and if there are no ticks, put one in Full for full permissions or Write if you only want write permissions.
If that isn't enough, the fix for this that works in some circumstances needs a Registry edit so go from Start to Run and into the open box, type regedit then press Enter. From the File menu, choose Export then note the name and location of the backup you're about to make - it may be crucial if anything goes wrong.
Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\StorageDevicePolicies and double click the key WriteProtect in the right pane and set the value to 0 In the Value Data Box and click OK. Leave the Registry from the File> Exit menu and restart the computer - twice.
Hopefully, one or the other will do the trick but post back if they don't.