How to determine that rootfs really exist on the USB disk after unslinging
This is taken (with the command df) from a freshly Unslung NSLU2 with 10GB USB disk. If the size of rootfs (9512992) and / on /dev/sda1 (which is my USB disk on port 1) reflects the size of your disk's data partition (/share/hdd/data), then you are good.
More readable disksizes: If you have a hard time interpreting the size and are more comfortable with Kbytes and Mbytes, use the command 'df -h' (h stands for human-readable). Sample output would look like this:
BTW, this is output of an OpenSlug 2.5-beta NSLU2.