I reinstalled a netbook's OS (win7 x86) and I'm trying to put on a separate partition a minimal Linux install, a compressed image of the system partition, and a script to restore it if need be. For this purpose, I installed the base system from a Debian Squeeze netinst, which only gives me the commandline and essential tools, but that ought to be everything I need.

After booting it, I mount the Windows partition:
mount /dev/sda1 /mnt

It mounts it with no errors, and mount has the following to report:
/dev/sda1 on /mnt type ntfs (rw)

However, creating any new files on the Windows partition, or copying files to it, returns a permission denied error.

The weird part is, if I edit a file already present in the partition (I tried editing config.sys with nano) and then save it, the changes persist - so it is capable of writing to the partition, but somehow it refuses to do so for new files.

Any ideas how to solve this? Hopefully without installing anything new, since this stupid netbook requires additional firmware for everything including ethernet?

I don't use Debian, but I'd be guessing you're using the in-kernel NTFS support. Try ntfs-3g.

Isn't the in-kernel ntfs support what it normally uses? Every normal (read: full desktop+gui) install of Debian I've done in the past few years has been able to mount NTFS properly... unless what it normally uses is ntfs-3g, but I doubt it because I've never even seen it used by default on my other Debian systems.

Last I looked (a couple/few years ago) the in-kernel driver couldn't handle new allocations properly.
Haven't tracked the development (if any) though.