Difference between Size and size on disk

[2ninup]

Hi,
I was looking at the size of a backup folder on my 6GB (2x3GB disks) ZyXEL NSA325v2 and the two readings were significantly different. The size showed 21.1GB but the size on disk shows 273GB. I put a screenshot of the properties on http://fradella.com/screenshot.jpg. Can anyone explain why there would be such a drastic difference.

Thanks for any help someone can offer,
Michael

 

[USAFRet]

.jpg not available.

 

[k1114]

Since it's drive Y, I'm thinking some removable media. Most likely allocation size.

 

[2ninup]

Thanks for the reply, it makes sense