http://www.tomshardware.co.uk/forum/249024-14-allocation-unit-size-formatting-500gb not only has exactly the same text including the same error (the K) as your current post, but also contains the explanation.

I leave it to you to think about what allocation unit size is best when "saving" small files (such as a website usually contains) or large files (such as video and pic files).
By the way, I assume that you already know that using an external disk (if this is an external one, you didn't tell) for STORAGE or SAVING isn't a good idea. Consider an external disk to be temporary storage, only good for moving files between different locations and as one of your backup devices.

Kees