How to make a disk dump using dd command?

To virtualize a physical PC in VirtualBox (see here), I need a copy of its disk image. The PC is quite old 386 (32bit) and its spec is quite low. So, I choose the less resource demanding Linux distro, Tiny Core (CorePlus version) to boot it up. To hold the big image file, I have a USB external drive in NTFS format. Here is what I did:

1. boot the PC with CorePlus

2. install ntfs-3g extension

3. connect the USB drive to the PC

4. find the device ID of the USB drive (in my case, sdd) with: fdisk -l

5. sudo -s

6. mount /mnt/sdd1

7. cd /mnt/sdd1/

8. dd if=/dev/sda of=sda_image.dd