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closed as too localized by bwDraco, Canadian Luke, soandos, 8088, ChrisF Nov 4 '12 at 11:23

This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

Note that the question could be read a 'any normal eSATA docking station'. If asked specifically about a single model then it is too localised for this site. – Hennes Nov 3 '12 at 18:50
up vote 1 down vote accepted

There is nothing magical about 3TB drives. There is a software limit where MSDOS style partition tables can't hold more than 2TB but that's not too important for an external disk, only if you want to use it as a boot drive. So yes, that and everything else that supports the SATA standard supports 3TB SATA drives. The problem is simply not on that level.

For a more precise answer, the so-called MBR partition table format has 4 bytes for the first sector and 4 bytes for the length of the partition. Now, the length is measured in sectors and the common sector size is 512 bytes. Calculate 2^32*512 = 2,199,023,255,552 bytes. WD have chosen to go with 4KB sector for some of its 3TB drives ("Advanced Format") cleverly sidestepping the problem for a few years to come (that is, until 16TB drives -- but new partition tables are spreading even now so likely it won't be a problem in 5-10 years when those come about). You would think this comes at the cost of wasting some space for <4KB files but that's not so because common (NTFS, HFS+ ) filesystems are storing things in 4KB blocks anyway. So that means you can even boot from large drives using 4KB sectors and you don't waste a thing.

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A good clean explanation. One minor flaw though: The WD advanced format drives do use 4k sectors but still tell the OS that they use 512 byte sectors. As a result the current WD advanced format drive suffer from the same 2TB limit. – Hennes Jul 22 '12 at 12:51
Really? That's bloody stupid! Why would they do that...? – chx Jul 22 '12 at 16:08
Compatability. The SATA specification has two fields, one to show the used (or emulated) sector size, one for the real sector size. Current WD AF drives report 512 bytes sectors when the first field is queried, and lie with a 512 byte answer when the second field is queried. – Hennes Jul 22 '12 at 16:14
If I plan to use this disk with the "toaster" on Mac, Linux, and Windows, what file system should I use? – qazwsx Jul 22 '12 at 16:54
That's a new question :) First you need to accept the answer here, second file a new question. – chx Jul 22 '12 at 17:12

"There is nothing magical about 3TB drives.". True, but there is as soon as you hook it up to a USB bridge chip which is the magic that facilitates connectivity.

A USB-attached device "speaks" the SCSI command set. This requires a protocol and command conversion. The problem is that since SCSI was first implemented decades ago, the most common commands, such as READ & WRITE allowed only 4 bytes for the address offset. Do some math, and FFFFFFFF hex blocks = 2.09TB.

This worked just fine until the early 2000s when people such as me who were writing RAID firmware and such and to start implementing 16-byte SCSI Commands, and companies such as microsoft had to start adding support for them in their O/S.

To make a long story short, MANY of the USB bridge manufacturers didn't even have to worry about their stuff breaking until disks got larger than 2TB, which is fairly recent when you consider how long it takes to turn a drawing and microcode of a chip into a finished product.

THe correct answer is that unless the manufacturer is quite proud that their device supports disks > 2TB, assume that it doesn't work. Also assume that if you open up any of these external bridges that there could be a different chip inside of one at any time in the future. I have seen this quite often, as we write code that does disk diagnostics, and we constantly see some vendors change the 'guts' of these things and it breaks code.

If you want to know for sure, then this is such a moving target that the only safe thing to do is ask the manufacturer if it supports the >2TB disks, and if there are any old versions out there with same part number that doesn't.

Now as for the WD 3TB disk. This is an "AF" or advanced format drive. DO NOT USE ONE OF THOSE disks unless you have both AHCI (which means forget attaching to USB), AND the appropriate device driver that rewrites the I/O for efficiency.

Technically you can use an AF drive behind a USB bridge, but performance will suffer. I suggest you just don't even try. Intel, as example even has firmware updates and driver updates for motherboards that use their SATA controllers that adds proper support for these disks.

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+1 for including a lot of details – PiotrK Aug 9 '12 at 19:00
+1 for an answer from someone who obviously actually knows what they are talking about. – Ausmith1 Sep 25 '13 at 13:48

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