I am running Windows 7 Ultimate 32 bit
but does it matter? because I am using VirtualBox..
Yes, it matters, in the sense that it is very likely that NO Dos drivers will be able to manage the Virtual hardware that VirtualBox provides. (I thought that I was clear on that and that you wanted to try on "real" hardware).
The few references I could find all seem like tales of horror:
If you want to access your USB hard disk from a VM, then it would be much easier if you run another VM, Qemu.
The Qemu + Qemu Manager running on a NT based system (such as 7):
allows to "hook" to the Virtual Machine a \\.\PhysicalDrive, which can well be your USB hard disk, the DOS won't need any USB driver, as the device will be presented to it as it was an internal hard disk.
It is possible to do the same in VirtualBox, by using a "physical" VMDK, but is more complex:
As a side note, remember that DOS can only access (without third party drivers) only FAT12/16 and FAT32 formatted partitions/volumes, not NTFS, and there could be issues connected to size of the hard disk, if bigger than 128 Gb).