Question usb + drive letter

I have a htc Kaiser.

the problem:
Running vista connect the usb cable
then it shows portable device with a name of the device.

what am i searching.
i want to map my sd card from my portable device to F H or what ever drive letter is free.

can somebody explain to me how to do this?
a problem i have is when i edit a file on the sd it can not be written back cause the drive letter is not mapped.