I recently had this problem, I've got two SATA discs and a internal USB card reader. I found that the card reader was being detected first and being assigned sda through sdd and sde & sdf were the discs. I felt rather uneasy that a failure to detect the card reader on boot would mess up the allocation of the discs. I ended up compiling USB as modules and modprobing at startup, that way the discs are always sda and sdb.

EDIT: Looking at Marks post, I might give that a try, I'm using LVM but I've kept my / on a normal partition.