Hard disk

Hello,

my computer has 2 hard disk one is 40gig and other is 20gig. my problem is when i try to transfer a file or document from the master harddisk to slave harddisk the slave harddisk is disappearing. and when i restart the computer the other disk is gone the computer don't show the other disk anymore. so what i am doing is i turn off the computer and i removed the ide cable and put it again but the problem is going back if i transfer a file.
thank you