Booted BT4r2 usb stick, started X, ran install.sh, after creating ext3fs on sdb1 (the target for the install). Manual partition setup, specified sdb1 for root and sdb3 for swap (already a swap partition for Arch Linux that boots on sda.) Ended up with this:

The installer needs to commit changes to partition tables,
but cannot do so because partitions on the following mount
points could not be unmounted:

/media/cdrom0

Please close any applications using these mount points.

Would you like the installer to try to unmount these partitions again?

Really? Commit what changes? I did not change the partitions at all, I just set sdb1 to be the root, "/", for this install. Also, unchecked install boot loader, as I'll just point the grub config on sda to boot sdb1 when "Backtrack" is selected in the grub boot menu.

I've a backup plan that will work, as it involves using Arch Linux, dd and a GBit connection and not ubiquity, however, I'm curious why no changes to the partition table have to be commited and why cdrom0 has anything to do with it? I checked mount in a terminal and found references to sda and hda, but no sdb's anywhere, and that's the only HD that should have been touched at all.