To make a partion writeable right-click on the icon of the partition and choose "mount" (i don't know the exact english option , because i use a german version) in the menue.
Afterwards there should be a small green triangle at the lower right corner of the icon.
then right-click again on the same icon and choose "change read/write status" or something like this.
You should get a window where you will be asked to change to writable or not.
when you are asked later, where to save a file after download,
use "/mnt/hd??/ ... /filename" (?? = see name of the chosen partition above).

When you save all your options to floppy/Harddisk after that, the Moozille-Download-Manager e.g. remembers the chosen path after the next reboot (with option myconfig=scan e.g.)

I don't know an easier way, but it works fine for me.