How I can assign sdc/sdd to my usb drives

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http://askubuntu.com – I am having strange problem. Can't find any solution. Using Ubuntu 12.04. My hard disk drive assigned as sda. If I plug-in any USB drive, it automatically assigned as sdb. That's fine. But after reboot, USB drives assigned as sda and hard disk drive shows as sdb. So my fstab mount point showing error on start. I would like to make any USB drive, goes after hard drive - as usually been in Windows. Please, anyone help me. I am struggling at this point. Thanking you in advance. Raihan (HowTos)