Yep, old floppies can ruin a drive pretty quickly - fungus can grow on the media if they're stored in even slightly damp conditions and that can come off on the heads, meaning they can't read or write any more. Also, the media itself can come off the plastic substrate and cover the heads, making them useless until cleaned gently with alcohol and cotton buds. This seems to be especially the case with cheap 'n' cheerful floppies...
