It's in general a bad idea to rely on the order of the drives. udev assigns them dynamically and the order might change.
File systems are better addressed by label or UUID:
/dev/disk/by-label/
/dev/disk/by-uuid/
Use the command `blkid` to find out information about your file systems.
Disks are addresses by id
/dev/disk/by-id/
If the disk ids are too ugly to work with you can symlink them to e.g. `/dev/mainhdd`, `/dev/backupdrive`, etc. via udev rules. Manual symlinks would not be persistent, since `/dev` is a kind of ram disk.