Hi, Ed.
As Paul explains in more detail below, the point is not just the capacity of
"such a large disc". As I found out after I got my 3 TB Seagate GoFlex Desk
external (USB 3.0) drive last year, it is a matter of physical limitations
of our old, familiar MBR's Partition Table and NTFS - and just plain
arithmetic with very large numbers. So we need a new file system, too, and
maybe a new or updated BIOS.
Quickie version: The MBR's Partition Table has room for exactly 4 partition
entries of exactly 16 bytes each, and a LOT of information is packed into
each of those bytes. Some of the bytes are divided into their individual
bits to allow more information to be coded. The first byte says whether the
partition is bootable; others hold the starting and ending sectors of the
partition. The last 4 of those 16 bytes hold a hexadecimal number
representing how many sectors are dedicated to that partition. MS-DOS and
earlier versions of Windows could use only the first 7.8 GB of a disk;
current versions combine those last 4 bytes to allow larger partitions. To
quote the online version of the Windows XP Professional Resource Kit
(
http://technet.microsoft.com/en-us/library/bb457122.aspx):
Using the Relative Sectors and Total Sectors fields (resulting in a 32-bit
number) provides eight more bits than the CHS scheme to represent the
total number of sectors. This allows you to create partitions that contain
up to 232 sectors. With a standard sector size of 512 bytes, the 32 bits
used to represent the Relative Sectors and Total Sectors fields translates
into a maximum partition size of 2 terabytes (or 2,199,023,255,552 bytes).
So the largest partition size we can have in most systems now is not a nice
round number; we usually speak of it as a "2 TB" or a "2.2 TB" limit, or
just "<3 TB", but none of these is really correct.
That's why disks larger than the 2.2 limit must use something newer than the
MBR system: The GPT, or GUID Partition Table file system. And not all
Windows versions - or all BIOSes - are ready to handle that yet. To go from
a 1 GB disk to 1 TB is no big deal; NTFS and MBR can handle that easily.
From 1 TB to 2 TB is just as easy. But to get to 3 TB is a big deal!
For larger disks, we have to use larger sectors so that we can address them
with smaller sector numbers. By converting from 512-byte sectors to 4 KB
(4,096) sectors, we can pack 8 times the number of bytes into the same
number of sectors. This alone would let us increase capacity to about 17.6
TB, but other changes allow the newest file system to handle many times
that.
I'm not a techie, Ed, so this is about the best I can do. As Paul Harvey
sometimes said, "Don't ask me for details. I've already told you MORE than
I know." But it might point some of the readers here to expert advice. And
it might be enough for some us for whom the simple explanation will suffice.
RC
-- --
R. C. White, CPA
San Marcos, TX
(e-mail address removed)
Microsoft Windows MVP (2002-2010)
Windows Live Mail 2012 (Build 16.4.3505.0912)) in Win8 Pro
"Ed Cryer" wrote in message
Will Win7 boot ok with such a large disc; and handle it well enough?
I've used a 2TB one with no problems.
XP used to hang when I tried to boot with a 1TB external drive plugged in.
Ed
