To access a SCSI device, you need to know two things: where it is logically located on your SCSI chain and what type of device it is.
Where it is on the chain determines what order it will appear in on the device list. Note that the SCSI ID is what is used to determine location on the chain; this ID will normally be between 0 and 6 (but can be between 0 and 15 if you have an Ultra-Wide SCSI controller).
What type of device it is determines how it is addressed.
/dev/sda is the first SCSI fixed disk (hard drive or removable hard drive) on the chain. /dev/sdb is the second SCSI fixed disk. To access partitions on hard drives, you follow the device name with the partition number (e.g. /dev/sda1 is the first partition on /dev/sda); you can access up to 128 drives and up to 15 partitions per drive.
/dev/scd0 is the first SCSI CD-ROM on the chain. /dev/scd1 is the second SCSI CD-ROM. You can have up to 256 SCSI CD-ROM drives.
/dev/st0 is the first SCSI tape drive on the chain. /dev/st1 is the second SCSI tape drive on the chain. You can access up to 32 tape drives.
/dev/sg0 is the first miscellaneous ("generic") SCSI device on the chain (most often this will be a scanner; it can also be a CD writer). /dev/sg1 is the second generic SCSI device. You can have up to 256 generic devices.
Note that to use an external SCSI device, it must be switched on when you boot the system. Also, it is a bad idea to swap removable fixed disks while the system is switched on (it is OK, however, to swap CD-ROMs and tapes, when they aren't mounted).
If you have multiple SCSI controllers, the device assignments will get horribly confusing (there is a logic to it, but it defied my powers of explanation); I recommend reading the boot messages to determine what device addresses are being assigned to each device.