I'm sure I read somewhere that the hx2755 would take a maximum of 4GB but I can't find it again. I did find this review though:
hx2755 Specifications
If you click Switch to Detailed View at the top of the page it then says:
CF I., CF II., MMC, SD, SDIO
Supports memory cards with
capacity of up to 2GB and may complies with some (not SD 2.0/HC) memory cards with higher capacity than 2GB
It sounds like it's maybe not supported but perhaps someone else will know differently.