Nikon D300 Complete Manual page 119

Alternatively, you can use the Format option on the SETUP
menu, but the method just documented is usually quicker,
and doesn't eat up battery power by lighting up the color
LCD.
1. Press the MENU button to show the menu system.
2. Use the Direction pad to navigate to the SETUP menu
(yellow wrench icon).
3. Use the Direction pad to navigate to the Format
memory card option and press the > key on the
Direction pad to select it.
4. Use the Direction pad to navigate to Yes and press
the OK button to select it.
Formatting a CompactFlash card "removes" all information
Note:
and images from the card. Always save your images to a
computer before formatting a card! I use the quotes around
"removes" because the image data isn't actually erased;
only the directory information that points to it is rewritten.
While it is possible to recover images immediately after
performing an in-camera format, it is a hassle to do, and
Thom Hogan's Complete Guide to the Nikon D300
V1.02
Page 119