Questions regarding 4K "Advanced Format" (AF) and storage drives


I have done a bit of research regarding this new 4K standard "Advanced Format" (AF) of storage drives. I must admit I am a bit lost at this stage.

According to Wikipedia (, these drives have a changed sector size of 4096 bytes (or more commonly known as 4K), instead of the more traditional 512 bytes sector format.

Currently I have 3 drives in my machine: 1X OCZ Vertex 3 SSD, 1X 1TB Seagate and 1X 500GB Seagate. This past weekend I purchased a 3TB Seagate which I will replace the 500GB drive. Since the new drive is of this AF variety, I decided to look into what it actually means.

According to Seagate, if one has a 64bit Windows 7 machine (which I have), along with a EUFI BIOS (which I have as well), one will be able to natively hook up and utilize all 2.7-odd TB of the disk in GPT mode for both a master boot disk as well as a secondary disk - which again is the case and no worries here.

When formatting the new drive, it defaults to an Allocation Unit Size of 4096 bytes (4K). After some investigation, I discovered that this "Allocation Unit Size" refers to the Cluster size and NOT the Sector size as I originally thought it to be. Even if I run checkdsk from CMD, it comes up with the same value of 4096 for bytes in each allocation unit - but again this refers to Cluster Size and NOT Sector size (

I did some further searching and found another command from CMD wmic partition get BlockSize, StartingOffset, Name, Index which according to one chap ( should show the actual Sector size. For all my drives it shows a BlockSize (Sector size) of only 512 bytes! I even confirmed this using other Acronis and Paragon tools.

I know that I did partition and format all my drives on this 64bit instance of Windows 7 and according to Seagate, Windows 7 should be "aware" of this AF standard.

From what I understand this AF standard relates to the Sectors and NOT the Clusters of disks. Should all my drives' Sectors values be 512 bytes or is this wrong? I have read a bit about disk misalignment, is this the case? Should I change anything on my side?

Many thanks. Looking forward to some helpful input.

7 answers Last reply
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  1. If a clean drive is formatted to 4k sector size it will be aligned correctly.
    The reason you see 512 sectors is cause all the Current AF drives have 512byte sector emulation support for legacy usage.
  2. kitsunestarwind said:
    If a clean drive is formatted to 4k sector size it will be aligned correctly.
    The reason you see 512 sectors is cause all the Current AF drives have 512byte sector emulation support for legacy usage.

    I am sure you meant: "If a clean drive is formatted to 4k Cluster size it will be aligned correctly."? Because when you format a drive it only gives you the option to set the Allocation Unit Size, which is the Cluster size.

    Can one run it in its native 4K sector mode? - no emulation?
  3. Sector size is the number of bytes in each group of data on the disk - it's part of the way the hardware is designed. So a 4K drive stores data in hardware groups of 4K bytes. But even these drives emulate 512-byte sector drives so that they can be used in equipment that doesn't understand the larger sectors (older operating systems, consumer electronics devices, USB enclosures, etc. etc.). To the best of my knowledge there aren't any bare drives on the market that actually expose their true 4K sector sizes to the host.

    A kind of exception to this is external USB drives. The USB protocol only supports sector numbers up to 4 billion - and as a result the maximum size drive you can have with 512-byte sectors is 2TB. To get beyond 2TB, the chipsets inside the external enclosures that convert USB to SATA take one of two approaches - they either divide the 3TB drive into two "virtual" drives of 1TB and 2TB, or they emulate a 4K-byte sector drive. This emulation works whether the drive itself is a 512-byte sector drive or a 4K sector drive that's emulating a 512-byte sector drive.

    The cluster size is something used by the file system, not the hardware. With the NTFS file system used with Windows, smallest possible cluster size for modern large hard drives is 4K bytes. This doesn't really have anything do to with the drive's sector size (other than always being an integral number of sectors), but is rather the unit of granularity that the file system uses to allocate space on the drive. "Advanced Format" 4K drives can emulate 512-sector drives efficiently because all file systems use a 4K-byte or larger clusters and so units of I/O that the OS issues to the drive are always for 4K bytes or more. As long as the file system is aligned so that the file system starts on a sector number that's a multiple of 8 then the drive never needs any extra I/O's to service the requests from the OS.

    The long and the short of it is this: Just be sure that you partition and format you hard drive using Windows 7 (which create partitions using the correct alignment) and you really don't need to worry about any of this stuff...
  4. From the second paragraph above:

    "... or they emulate a 4K-byte sector drive"

    As I understand it then the disk emulates 512b sectors which the USB to SATA bridge then emulates back to 4kb sectors. This may seem trivial, but why the back-and-forth emulation? Why doesn't the disk just advertise itself with its native 4kb sectors in the first place (which the usb to sata interface could pass on)? Any insight would be appreciated.

    I suspect this has to do with USB access (SCSI) vs. SATA (ATA) access to the drive. SCSI apparently supports 4kb sectors whereas ATA doesn't.
  5. It's quite a stupid fuss yes. All I know up till now is you need (for Windows 7) updated drivers and tools which you can install by means of a hotfix. the plain MSDN Win7 install iso does not have this. Or you need to install the SP1 which has this fix as well. This will clearly report 4k sectors for a drive to software. Which is the answer to the poster above me I think. There's still many software that can only submit data in chunks of 512 B (I think)
  6. Hi Daniel - I have WIN 7 Pro SP1 and installed the hotfix and ... (BUZZER!) it didn't work. Does I worked with MS support, Seagate Support, HP Support and Acronis Support. Guess what? They ALL indicated (admitted) that sometimes it doesn't work. NO FIX, FULL STOP. GRRRRRRRRRRRRRRR....
  7. bios must also support the large drive sizes.
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