Go to setting-> SD Card and phone storage

if there a info on secondary storage . you are on it (app2sd). if not. you will have to partition your sd manually.

simple enough, but do a backup is a MUST

if on windows, use software like Partition Magic. take out your SD card (unmount first) and put in memory card reader.
1. backup
2. resize accordingly (FAT32, EXT3, linux-swap(optional)
3. put your sd card back to your HERO.
4. Power on and check the settings above

if on linux (like me), install gparted. connect to usb cable.
1. backup
2. mount sd on phone first to be recognized
3. launch gparted
4. unmount first
5. resize accordingly
6. apply and reboot.