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Not much to do here in Indiana at 6 in the morning, so I decided to decipher hard drive model numbers for my own education. Learned a lot in the process, here's the results. The data presented here is only for ATA drives, someone else can dig up the data on SCSI 
Also, this post is long enough with just half way current drives, so you're on your own for older drives
Conventions used in this post:
w, x, y, and z are numerical values, a, b, and c are for letters.
Hitachi
HDSwxyabcz0
HDS refers to Hitachi DeskStar
w = the spindle speed of the disk. Only the first two digits (72 for a 7200RPM drive) are given
x = the capacity, in gigabytes, of the largest member of the drive family, divided by 10.
y = the capacity, in gigabytes, of the drive, divided by 10.
a = Generation code, although this value can also be used to identify the maximum number of platters in the drive family
- P = 1
- D = 2
- V = 3
- K = 5
b = L, which stands for a low profile 3.5x1" form factor drive.
c = the drive's interface; AT= PATA, SA=SATA, A3=SATA 3.0 gbps
z = the size of the disks buffer; 2 for 2MB buffer models, 8 for 8MB and 6 for 16MB.
0 = Reserved for future use by Hitachi.
Note on Deskstar Families: You may sometimes have to use the values for 'a' as well as 'x' to determine which family a drive is from. For example, drives from the 7K250 and T7K250 families will have the same value for 'x' (25) but different platter counts 'a.' A T7K250 drive will have a 'D' for its two platters, while a 7K250 drive will have a 'V' for its maximum of 3 platters. On the flip side, a 7K400 and 7K500 drive will have the same value for 'a' (K) but different values for 'x.'
Example: HDS722516VLSA80 is a 7200RPM 160GB SATA drive, from the 7K250 family that has an 8MB buffer.
Maxtor
xayb0
xa = drive family
- 6Y = DiamondMax(DM) Plus 9
- 6B = DM10
- 6L = DM10 RoHS compliant version
- 6H = DM11
- 7B = MaxLine(ML)III
- 7L = MLIII RoHS compliant
- 7H = ML Pro
- 4R = DM16
- 2F = Fireball 3
- 6E = DM Plus 8
- 5A = ML II
- 7Y = ML Plus II
y = the capacity of the disk, in gigabytes. Drives less than 100GB will have a leading zero (80GB is denoted as 080, etc)
b = buffer size and interface
- J = 2MB PATA with ball-bearing motor
- L = 2MB PATA with fluid dynamic bearing (FDB) motor
- P = 8MB PATA
- R = 16MB PATA
- M = 8MB SATA
- S = 16MB SATA
- F = 16MB SATA 3gbps
All Maxtor SKUs have a zero at the end. It may have a value, but I'm not aware of one.
Example: A Maxtor drive with a model number of 6Y200P0 is a 200GB 7200RPM PATA drive with an 8MB buffer from the DiamondMax Plus 9 family.
Samsung
SPxyza
SP refers to SpinPoint, the brand name of all Samsung HDDs
x = Capacity in GB / 10
y = 0 or 1, Only applies to PATA drives; 0 for 2MB buffer, 1 for 8MB. All Samsung SATA drives have 8MB buffers (some of Samsung's newer SATA SKUs show a zero here despite their 8MB buffer)
z = number of active recording heads; 2, 3, or 4 depending on the geometry of the drive.
a = Interface; N for PATA drives, C for SATA drives.
Example: SP1213C is a 120GB SATA drive, with 8MB buffer, and 3 active recording surfaces (both sides of one platter and one side of a second platter)
Seagate
STxya
ST stands for Seagate Technology
x = 3, which means the drive is a 3.5" wide 1" thick low profile drive (all of Seagate's desktop drives ship in this form factor)
y = the capacity of the drive in megabytes
a = Interface; A = PATA and AS = SATA
Example: ST3160023A is a 3.5x1 inch PATA drive with a capacity of 160023MB, which when divided by 1000 to convert to gigabytes, comes out to a retail capacity of 160GB.
Western Digital
WDx0ab
WD stands for Western Digital
x = the capacity of the drive in GB
WD puts an additional zero after the capacity value
a = buffer size and spindle speed
- B = 2MB 7200RPM
- J = 8MB 7200RPM
- K = 16MB 7200RPM
- G = 8MB 10,000RPM
- S = 8MB 7200RPM RAID Edition
- Y = 16MB 7200RPM RAID Edition 2
b = interface of the drive
- B = PATA
- D = 1.5 gbps SATA
- R = 1.5 gbps SATA w/ NCQ
- S = 3 gbps SATA
Example: WD740GD is 74GB 10,000RPM SATA 1.5 gbps drive with an 8MB buffer, and WD2000BB is a 200GB 7200RPM PATA drive with a 2MB buffer.
Also, this post is long enough with just half way current drives, so you're on your own for older drives Conventions used in this post:
w, x, y, and z are numerical values, a, b, and c are for letters.
Hitachi
HDSwxyabcz0
HDS refers to Hitachi DeskStar
w = the spindle speed of the disk. Only the first two digits (72 for a 7200RPM drive) are given
x = the capacity, in gigabytes, of the largest member of the drive family, divided by 10.
y = the capacity, in gigabytes, of the drive, divided by 10.
a = Generation code, although this value can also be used to identify the maximum number of platters in the drive family
- P = 1
- D = 2
- V = 3
- K = 5
b = L, which stands for a low profile 3.5x1" form factor drive.
c = the drive's interface; AT= PATA, SA=SATA, A3=SATA 3.0 gbps
z = the size of the disks buffer; 2 for 2MB buffer models, 8 for 8MB and 6 for 16MB.
0 = Reserved for future use by Hitachi.
Note on Deskstar Families: You may sometimes have to use the values for 'a' as well as 'x' to determine which family a drive is from. For example, drives from the 7K250 and T7K250 families will have the same value for 'x' (25) but different platter counts 'a.' A T7K250 drive will have a 'D' for its two platters, while a 7K250 drive will have a 'V' for its maximum of 3 platters. On the flip side, a 7K400 and 7K500 drive will have the same value for 'a' (K) but different values for 'x.'
Example: HDS722516VLSA80 is a 7200RPM 160GB SATA drive, from the 7K250 family that has an 8MB buffer.
Maxtor
xayb0
xa = drive family
- 6Y = DiamondMax(DM) Plus 9
- 6B = DM10
- 6L = DM10 RoHS compliant version
- 6H = DM11
- 7B = MaxLine(ML)III
- 7L = MLIII RoHS compliant
- 7H = ML Pro
- 4R = DM16
- 2F = Fireball 3
- 6E = DM Plus 8
- 5A = ML II
- 7Y = ML Plus II
y = the capacity of the disk, in gigabytes. Drives less than 100GB will have a leading zero (80GB is denoted as 080, etc)
b = buffer size and interface
- J = 2MB PATA with ball-bearing motor
- L = 2MB PATA with fluid dynamic bearing (FDB) motor
- P = 8MB PATA
- R = 16MB PATA
- M = 8MB SATA
- S = 16MB SATA
- F = 16MB SATA 3gbps
All Maxtor SKUs have a zero at the end. It may have a value, but I'm not aware of one.
Example: A Maxtor drive with a model number of 6Y200P0 is a 200GB 7200RPM PATA drive with an 8MB buffer from the DiamondMax Plus 9 family.
Samsung
SPxyza
SP refers to SpinPoint, the brand name of all Samsung HDDs
x = Capacity in GB / 10
y = 0 or 1, Only applies to PATA drives; 0 for 2MB buffer, 1 for 8MB. All Samsung SATA drives have 8MB buffers (some of Samsung's newer SATA SKUs show a zero here despite their 8MB buffer)
z = number of active recording heads; 2, 3, or 4 depending on the geometry of the drive.
a = Interface; N for PATA drives, C for SATA drives.
Example: SP1213C is a 120GB SATA drive, with 8MB buffer, and 3 active recording surfaces (both sides of one platter and one side of a second platter)
Seagate
STxya
ST stands for Seagate Technology
x = 3, which means the drive is a 3.5" wide 1" thick low profile drive (all of Seagate's desktop drives ship in this form factor)
y = the capacity of the drive in megabytes
a = Interface; A = PATA and AS = SATA
Example: ST3160023A is a 3.5x1 inch PATA drive with a capacity of 160023MB, which when divided by 1000 to convert to gigabytes, comes out to a retail capacity of 160GB.
Western Digital
WDx0ab
WD stands for Western Digital
x = the capacity of the drive in GB
WD puts an additional zero after the capacity value
a = buffer size and spindle speed
- B = 2MB 7200RPM
- J = 8MB 7200RPM
- K = 16MB 7200RPM
- G = 8MB 10,000RPM
- S = 8MB 7200RPM RAID Edition
- Y = 16MB 7200RPM RAID Edition 2
b = interface of the drive
- B = PATA
- D = 1.5 gbps SATA
- R = 1.5 gbps SATA w/ NCQ
- S = 3 gbps SATA
Example: WD740GD is 74GB 10,000RPM SATA 1.5 gbps drive with an 8MB buffer, and WD2000BB is a 200GB 7200RPM PATA drive with a 2MB buffer.