Somehow both my (portable) external hard drive and one of my usbs ended up 'write protected' according to my computer; (though the external hard drive works fine at work/on my work computer).
Niether of them has a switch or anything. Just the standard ports.
Are your external storage devices using a Microsoft NTFS file system, and is your problem computer a Mac? If so, then be aware that Macs can read an NTFS file system but cannot natively write to it. One possible solution is NTFS-3G.