The partitions on each disk are represented by appending a number to the disk name. For example, the names hda1 and hda2 represent the first and second partitions of the first IDE disk drive in your system. Linux represents the primary partitions with the drive name plus the numbers 1 through 4. For example, the first primary partition on the first IDE drive is /dev/hda1. The logical partitions are numbered starting at 5, so the first logical partition on that same drive is /dev/hda5. Remember that the extended partition—that is, the primary partition holding the logical partitions—is not usable by itself. This applies to SCSI drives as well as IDE drives.
Let’s assume you have a system with two SCSI disks, one at SCSI address 2 and the other at SCSI address 4. The first disk (at address 2) is then named sda and the second sdb. If the sda drive has three partitions on it, these will be named sda1, sda2, and sda3. The same applies to the sdb disk and its partitions. Note that if you have two SCSI host bus adapters (i.e., controllers), the order of the drives can get confusing. The best solution in this case is to watch the boot messages, assuming you know the drive models.
As described above, you should have a separate smaller root partition and a larger /usr partition if you have the space. For most users, the two partitions initially mentioned are sufficient. This is especially appropriate when you have a single small disk, because creating lots of partitions can waste space.
In some cases, you might need a separate /usr/local partition if you plan to install many programs that are not part of the Debian distribution. If your machine will be a mail server, you may need to make /var/spool/mail a separate partition. Putting /tmp on its own 20 to 32MB partition, for instance, is a good idea. If you are setting up a server with lots of user accounts, it’s generally good to have a separate, large /home partition to store user home directories. In general, the partitioning situation varies from computer to computer depending on its uses.
For very complex systems, you should see the Multi Disk HOWTO[8]. It contains in-depth information, mostly of interest to people setting up servers.