[Help-ASAP] Why it is not avilable to choose different cache

i have 2 hdd 15gb (system 3)
40gb empty

both of them are connected to pc , the system-drive just used 100mb of the 15 gb!
and it is not an option to choose the other drive for cache !
any help please ??

go to Stores menu

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thanks Mr. Normis , you are the MAN