Look at this link:
http://www.cisco.com/en/US/products/hw/routers/ps259/products_configuration_guide_chapter09186a008007e5fb.html#wp1006123
If your flash is type B or type C it should be doable. Do not use "?" as it might be hidden just:
Directory Operations for Class C Flash File System
Create a New Directory
To create a directory in compact Flash memory, use the mkdir {flash: | slot0:} command.
The following example shows output for first displaying the contents of an internal compact Flash card, and then creating a directory named config and a subdirectory named test-config:
Router# dir flash:
Directory of flash:/
3 -rw- 6458208 Mar 01 1993 00:04:08 c3725-i-mz.tmp
128094208 bytes total (121634816 bytes free)
Router# mkdir flash:/config
Create directory filename [config]?
Created dir flash:/config
Router# mkdir flash:/config/test-config
Create directory filename [/config/test-config]?
Created dir flash:/config/test-config
Router# dir flash:
I suggest formatting it first.
