Did you cfdisk it, mkfs.ext3 it and then poweroff, boot back up
and then install?
Or did it all at once?
I don't know much 'bout Vbox, though I do use it.
You can practice with some portables though...
You could try Portable-Slackware
just stay away from KDE....it'll run pretty slow with KDE
just make a 8GB or whatever qemu-img with portable-qemu and use the scripts, alreadyset
for slack to install and run.
I am speaking of Portable-Qemu.tar.gz
I also have a pre-made XFCE4 portable-slack there too; but it
will de-compress to 3 or 4GB after untaring so be on notice
If you just use portable-qemu it's already setup for slackware
with scripts for windows and linux , a vhd maker a how-to and
all you gotta add is slackDVD