Yes, it is most likely due to the filesystem. I recommend you run fsck with the command:
This will correct any errors you may have in the filesystem. "fsck" should automatically detect the type of filesystem, by the way. If you can, please post the type of filesystem that is on the USB disk. Sometimes, NTFS formatted drives are mounted read-only. In this case, you should either force it to mount read-write (this may need to be enabled in the kernel), or just reformat the drive to a different filesystem. To reformat the drive as FAT32, use:
To format it as EXT3, use:
Hope this helps!