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Some manufacturers of external hard drives with capacity exceeding MBR partition capacity seem to sell them pre-partitioned to a 2TiB partition and one with the remaining capacity.

Tools are available to make a single, larger partition out of these (at least I think I learned as much from amazon reviews of such products; presumably the disk is in GPT mode afterwards).

Can I go the other way, i.e. use the full space of a 3TB drive, while at least one partition can be used in MBR mode (for booting Windows in a non-UEFI machine)?

The drive is a 3.5" HD Seagate Barracuda something currently inside a USB 3 box. This Seagate Disk Wizard (pdf) for Windows, which may not be helpful anyway, refuses to install for alleged lack of a Seagate HD. I have a (non-UEFI) PC (with SATA I only) available and would not be afraid to use linux-based tools.

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I don’t think Windows would be very happy with booting from a hybrid MBR/GPT drive. I haven’t actually tested it, though. – Daniel B May 24 '14 at 12:40
If I understood some not terribly well-sourced article correctly, the original trick had something to do with making the second partition appear as a virtual hard drive on its own, so no GPT required. – arne.b May 24 '14 at 12:45
What are you attempting to do once the drive is partitioned. Perhaps we can recommend a simpler alternative... – Big Chris May 24 '14 at 12:46
That functionality depends on the external disk’s USB-SATA bridge controller. If you plan to install Windows on it, you would need to install it internally. The splitting functionality won’t work in that case. – Daniel B May 24 '14 at 12:46
@BigChris Use it as a drive for Windows and other stuff. Put backup image of old hard disk somewhere at the far end of the disk. An obvious simpler alternative would be to live with a 2TiB disk until an UEFI PC comes around, but I wondered if I could do better. – arne.b May 24 '14 at 12:54

MBR or GPT is the whole harddisk's format, not a partition, so you cannot have an MBR partition on a GPT disk. Hybrid MBR/GPT is a drive with both GPT entries and normal MBR entries, not MBR partition on GPT disk too

Some manufacturers do a workaround by partitioning the first 2TB then install windows just like normal. After finish installing they install a special driver to make the OS recognize the drive as 2 drives, then you can use the remaining space. But I don't like/recommend using it this way.

You can boot from GPT disks in BIOS mode using DUET, although not very easy but if you want you can reference the information here

Another solution is installing windows on VHD on a GPT disk. Just format the drive as GPT then when installing windows, press shift+F10 and create a VHD, mount it and install windows as normal. I haven't tried it but many people has confirmed that it works

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