can't amend/change files wirelessly on external drive, why?

[antonioconte]

Hi, I'm trying to work on my macbook pro wirelessly and trying to access an external drive which is connected to an imac. I connect and I mount the drive on my desktop, I can browse and open the files but for some reason I can't save? Even when I open a dreamweaver file, it says "you can open this file but you can't change it"

any ideas?

Oh, the imac is running 10.4.8 too.