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When you right click on the unallocated partition you can not do a format or something? Are you receiving an error or what is a problem?
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Some systems have a 2TB limit for the 1st disk. You should use a max 2TB HDD as the 1st disk and use the 3TB HDD as the 2nd.
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@helpfinder no option for that<br />
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@strivoli ok i will try
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Is this the only disk in your PC, or is the OS on another disk? If the OS is on another disk, convert the 3TB disk to GPT within Windows Diskmanagement (this will delete all data on it, so make sure your backups are good). Once it has been converted to a GPT disk, you will be able to use all the space.<br />
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If this is the only disk in your PC, and the PC's BIOS isn't of the new UEFI type, and your Windows OS isn't 64 bit, you will not be able to use it's full size. Windows can only boot from an MBR disk on non UEFI systems, and MBR disks are limited to 2TB in size. Linux doesn't have that limitation, so you could install Linux to a GPT disk even if you don't have an UEFI BIOS.
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What's GPT, sorry?<br />
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I have done that and it works now.
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Apparently, the screenshot shows no anomalies. What happens when you right-click on the unallocated 746GB partition? What options do you see, what options are available? Can you post the screenshot of right-click menu? What error do you receive when you try to create a partition?<br />
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I see that there is no data on the two newly created partitions (D and E). If you don't have any (valuable) data on those two partitions (if you do, please backup data first) you could delete D and E partitions and make only one partition. If you can create just one partition, delete this one partition you have just created and try to create three partitions.
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GPT replaces the old MBR disk organization. MBR is limited to 2TB size, and also the maximum number of primary partitions on MBR disks is 4. GPT doesn't have those limits.
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<div class="content wysiwyg-content">
Dear experts!<br />
<br />
On my PC, I bought a 3TB drive and partition them as <br />
<br />
D: 1TB<br />
E: 1TB<br />
<br />
There seems to be 746GB unallocated which I can't assign a drive or allocate or format.<br />
<br />
Please advise (screenshot attached)<br />
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Presentation1.pptx

Dear experts!

On my PC, I bought a 3TB drive and partition them as 

D: 1TB
E: 1TB

There seems to be 746GB unallocated which I can't assign a drive or allocate or format.

Please advise (screenshot attached)

3TB - Wasted storage issue

Storage devices include any device used for storing and retrieving digital information. Hard disk drives (HDDs) use one or more rigid ("hard") rapidly rotating disks (platters) coated with magnetic material. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored or retrieved in any order and not only sequentially. The primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs), but HDDs remain the dominant medium for secondary storage due to advantages in price per bit and per-device recording capacity. CD-RW (Compact Disc-ReWritable) is a digital optical disc storage format that allows information to be stored outside the mechanical HDD.

Storage Hardware

Computer data storage, often called storage or memory, is a technology consisting of computer components and recording media used to retain digital data. In addition to local storage devices like CD and DVD readers, hard drives and flash drives, solid state drives can hold enormous amounts of data in a very small device. Cloud services and other new forms of remote storage also add to the capacity of devices and their ability to access more data without building additional data storage into a device.

Storage

Windows 7 is an operating system from Microsoft. Features include multi-touch support, a redesigned Windows Shell with a new taskbar, referred to as the Superbar, a home networking system called HomeGroup, and performance improvements.