[Grml] Writing to Hard Drives from LiveCD

I have observed something odd: When I write data to a mounted hard drive partition while running from the livecd, the amount of written data is subtracted from the free RAM, i.e., if I do a

dd if=/dev/zero of=/mnt/hda1/tmp/testfile bs=1M count=512

with /dev/hda1 mounted on /mnt/hda1, I am losing 512M of my RAM; deleting the file frees up that RAM. After rebooting, the file is still there, but doesn't take up any RAM anymore.

I think I may be missing something vitally important about how the unionfs is set up. Can anybody please tell me how I can write files to an HD without them taking up RAM, too?

Any help would be much appreciated! Cheers, --j