Which hard disk drive is which?

Disk Utility

This is a GUI application that will give you information about the model, size, serial number and device path of your drives. It is installed by default on Ubuntu Desktop. You can press the Ubuntu Dash button and search "disk utility" to find it, or run it in the terminal with palimpsest.

hdparm -i /dev/sdX gives you the serial number, which is the simplest way I know of to tell apart hard disks of the same brand and size.

The serial number is normally printed on a label on the disk, so although you need to open the case to find it, there's no need disassemble the computer.

Example:

$ sudo hdparm -i /dev/sdb | grep -i serial
 Model=SAMSUNG HD253GJ, FwRev=1AJ10001, SerialNo=S24JJ90Z505435

If you want more info lshw -c storage -c disk gives the most readable output. It's pretty much the same data as Disk Utility, just in command line format.

Here is an annotated example from the most complex setup I have access to, with four disk controllers, seven hard disks, a DVD ROM and a USB disk.

The output has been cut down to size to focus on the interesting parts:

[server ~]$ sudo lshw -c storage -c disk
  *-storage   
  # a 2-port PCI-E SATA controller
       description: SATA controller
       product: 88SE9123 PCIe SATA 6.0 Gb/s controller
       vendor: Marvell Technology Group Ltd.
     *-disk
          description: ATA Disk
          product: WDC WD15EADS-00P
          vendor: Western Digital
  # 'physical id' corresponds to port number, first port is 0
          physical id: 0  
          logical name: /dev/sdg
          serial: WD-WMAVU0849124
          size: 1397GiB (1500GB)
     *-cdrom
          description: DVD reader
          product: BD-ROM BR-5100S
          vendor: Optiarc
          physical id: 1
          logical name: /dev/cdrom1
          logical name: /dev/dvd1
          logical name: /dev/scd0
          serial: [Optiarc BD-ROM BR-5100S 1.02 May20 ,2008
  *-storage
  # mobo controller for eSATA ports. Not used.
       description: SATA controller
       product: JMB362/JMB363 Serial ATA Controller
       vendor: JMicron Technology Corp.
  *-ide
  # mobo controller for IDE. Not used.
       description: IDE interface
       product: JMB362/JMB363 Serial ATA Controller
       vendor: JMicron Technology Corp.
  *-storage
  # the primary SATA controller, six ports
       description: SATA controller
       product: 82801JI (ICH10 Family) SATA AHCI Controller
       vendor: Intel Corporation
  # 'disk:0' means port 0, same as physical id
     *-disk:0
          description: ATA Disk
          product: WDC WD1600BEVS-0
          vendor: Western Digital
          physical id: 0
          logical name: /dev/sda
          serial: WD-WXEY08T58317
          size: 149GiB (160GB)
     *-disk:1
          description: ATA Disk
          product: ST32000542AS
          vendor: Seagate
          physical id: 1
          logical name: /dev/sdb
          serial: 5XW1RTDS
          size: 1863GiB (2TB)
     *-disk:2
          description: ATA Disk
          product: ST32000542AS
          vendor: Seagate
          physical id: 2
          logical name: /dev/sdc
          serial: 5XW23W0W
          size: 1863GiB (2TB)
     [...]
  *-scsi
  # the USB disk, as evidenced by the bus info 
       physical id: 1
       bus info: usb@1:1
       capabilities: emulated scsi-host
     *-disk
          description: SCSI Disk
          product: 10EAVS External
          vendor: WD
          physical id: 0.0.0
          logical name: /dev/sdh
          serial: WD-WCAU46029507
          size: 931GiB (1TB)

I think the Disk Utility is faster to read; the command line version has the advantage of being usable over ssh and in scripts.

And I still think the serial number is the most reliable solution :)

The "Physical ID" of each drive corresponds to where it is connected to the motherboard. and starts with 0. so a physical ID of 2 would mean that the drive is attached to the 3rd sata port of your mobo or other applicable device.

Open the case and listen to the drives with a rolled up copy of "The Linux Journal". Then run a dd on each drive in turn to generate some disk noise.