Asus wl-500w USB storage sharing problem

Hi!
I have a problem, when trying to share my HDD on the router. As described in user manual, I configure it like on the picture below, giving the permission to "Guest" to read/write:
http://www.shrani.si/f/28/11f/36U7kWH8/router.png

I see the disc and files on it, but I don't have writing permission. I've tried everything, but still, no success. I always get message like that:
http://www.shrani.si/f/2s/kw/1U2A2cXI/router.png

Can any of you guys help me, please? :)

it could be that the file system is not writeable.
The router can only read/write FAT32 and EXT2

NTFS is read only

Same problem on WL500W, which should have USB 2.0, but I only get around 2500 KB/s when copying files onto/from my attached external HDD.

Anyone knows, why it can't do better?