Can't edit files on SD Card when mounted on PC

[ccc3]

I managed to mount the phone on a Windows XP and I see the contents of the card in
My Computer\GT-I9300\Card

I can copy files to and from the card, but if I open a file straight from the card in order to edit it, it can't be saved directly. Instead I have to save it on the computer and then transfer it to the card, overriding the old one, which seems tedious.

Is this the normal behavior ?

I would have expected the same functionality as with the old "mount as USB", where the files on the card can be directly edited, as if they were on an external disk.

Thanks.