When I wanted to create multiple partitions on a USB stick, I found a way to make windows think it's local disk...
using with the cfadisk.inf driver
Perhaps if you chose "load driver" during windows setup and loaded this driver for your usb hard disk, it might work (I haven't tested it so far)
You need to edit cfadisk.inf and change the DEV/VEN lines beforehand according to your SDHC's hadrware ID.
Here's a detailed guide if you wanna give it a try: MSFN Forums > A Multiple Partition USB Stick with Multi Boot OS
On the other hand, I think in some recent BIOSes there is an option to force the USB to fixed disk mode, floppy mode etc..If you have this option in your BIOS, give it a try.
I will try the first method when I have time and let you know how it goes.