How Do Those 4TB External Hard Drives Have One Partition Yet Work In XP?

rival1

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Dec 28, 2013
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Hello,
Usually I'm able to answer my own questions by looking at the forums here and other places as well as searching online; but this one seems elude me. I'm going to focus specifically on the Seagate 3TB, 4TB, and 5TB External Backup Plus Hard Drives. When you plug one of these into a USB port, on a XP machine, you will see the entire capacity of the hard drive available to you, as a single partition. No drivers are necessary, not even a reboot and yet it's all there. Now, if you buy a 4TB internal hard drive, you can get DiscWizard software that will make the entire 4TB (or whatever) hard drive available to use, but it will make a 2.1TB partition and make another partition for the rest of the space if you stick it in a XP machine.
There seemed to be a Seagate forum post asking this question some time ago, but Seagate has done away with their forums, because when you click on a link to go to one of these posts, it just redirects you to community.seagate.com and searching from their page yields no results. I have an idea that this has to do with the Advanced Format technology that has allowed the manufacturers to make hard drives with 4k physical sectors, but I'm just guessing. Also, according to Microsoft itself, 4k physical sector hard drives are only supported in Windows 8 and Windows Server 2012, not even in Windows 7. I was wondering if someone in the community could answer just how these external hard drives then work with Windows XP? As a secondary question, why can't this technique then be used on internal hard drives? Thanks in advance for your thoughts.
 
Solution
The 2TiB limit for an MBR partition is not a capacity limit per se. Instead it reflects the maximum number of LBAs that can fit into the 32-bit doubleword that is reserved for this parameter in the partition table.

The capacity is given by ...

(number of LBAs) x (bytes per LBA)

When the drive is connected directly via SATA, it presents a sector size of 512 bytes.

The max capacity of an MBR partition is then ...

2^32 x 512 = 2TiB

http://www.google.com/search?q=2%5E32+x+512+bytes+in+TiB

When the drive is inside the enclosure, it becomes a "USB mass storage device" and the firmware on the USB-SATA bridge board then presents a sector size of 4096 bytes to the host.

The max capacity of an MBR partition then becomes ...

2^32 x 4096 =...
Hi there
Check the file format of the external. Most likely it is formatted as FAT32 for compatibility reasons. This does lead to large data clusters and wasted space on the drive.
When XP wants to partition the drive, it will want to use NTFS as the file system. Xp, however, does not support GPT partitions, which is what you need to use to partition those large drives.
Hope this clarifies things a bit.
 
The 2TiB limit for an MBR partition is not a capacity limit per se. Instead it reflects the maximum number of LBAs that can fit into the 32-bit doubleword that is reserved for this parameter in the partition table.

The capacity is given by ...

(number of LBAs) x (bytes per LBA)

When the drive is connected directly via SATA, it presents a sector size of 512 bytes.

The max capacity of an MBR partition is then ...

2^32 x 512 = 2TiB

http://www.google.com/search?q=2%5E32+x+512+bytes+in+TiB

When the drive is inside the enclosure, it becomes a "USB mass storage device" and the firmware on the USB-SATA bridge board then presents a sector size of 4096 bytes to the host.

The max capacity of an MBR partition then becomes ...

2^32 x 4096 = 16TiB

 
Solution

rival1

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Dec 28, 2013
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Thank you for this, I believe you hit the nail on the head. The controller must be doing some sort of sector translation. As a confirmation, I took the HD out of its enclosure and put it directly into my machine. All my data was unreadable, and it looked like an uninitialized, unpartitioned, and unformatted hard drive. Put it back in the enclosure and all the data was again accessible. My only remaining question is if this is controller-based magic, then why can't they do this on the controller board of the hard drive itself? Is there something special about a USB-SATA bridge that the controller board on the HD itself can't emulate in regards to presenting 4k sectors that is different than an AF 4k native hard drive?