See if you can see it in my Computer (if it has a letter assigned to it). you can open it there. If Avira locked it, you can try to unlock it by doing a right-click on it> properties>security and Allow the Write Permission. That should work.
If not, you can try and do a format in Disk Manager. Beware, you will lose the data that's on the disk.
It might need to be "initialized" first, which is a low-level format. Then you will be able to format it in whatever filesystem you prefer (NTFS, FAT, etc.). this also will erase all data on it.