The box said 3 TB. Disk Management says you may use about 2 TB, with a large slab of "unallocated" space that refuses every right-click you throw at it. Your disk is not damaged and the seller did not cheat you. It was initialised with the one partition style that cannot count past 2 TB.
MBR was designed when a big hard disk meant megabytes. Its partition table stores disk addresses in a way that runs out of numbers at 2 TB (technically 2^32 sectors of 512 bytes, about 2.2 TB, shown by Windows as roughly 2 TB usable). Anything beyond that line is not broken — it is unaddressable: the table literally cannot point at it, so Windows shows it as unallocated space you cannot extend into or format. GPT has no such ceiling at any size you can buy. The fix is to convert the disk to GPT — the only question is whether the disk already holds data.
👆 Example values. The grey unallocated slab past the 2 TB line is the signature — it ignores every format attempt.
diskmgmt.msc) and look at the disk: one usable area up to ~2 TB plus a grey unallocated area you cannot useTwo false alarms to rule out: a "3 TB" drive honestly shows as about 2794 GB total (makers count in thousands, Windows counts in 1024s) — that difference is normal and is not this problem. This problem is specifically the unusable slab beyond 2 TB.
The convert option is greyed out while any partition exists on the disk — that is Windows protecting data, not a fault. An uninitialised brand-new disk simply asks on first use: choose GPT at that prompt and the problem never happens.
The Disk Management convert route requires an empty disk, so for a drive already holding files the order is:
One exception: if this is your Windows system disk, the built-in MBR2GPT tool converts it in place without data loss — that route has its own page in this series. Third-party tools offer in-place conversion for data disks too; they are generally fine, but the backup-first rule applies to them exactly the same.
If the drive sits in an old USB enclosure or dock, test before blaming the partition style twice: some cheap enclosures themselves cap at 2 TB or translate sector sizes in ways that make partitions created in the dock unreadable in a PC, and vice versa. If a GPT conversion inside the enclosure still misbehaves, connect the drive directly to a SATA port, convert and format there, and only then consider the enclosure — and honestly, a 3 TB drive deserves a modern enclosure anyway.
The video explains the MBR 2 TB limit with the disk-size example on screen — two minutes of it makes this entire problem obvious at a glance next time.
▶️ Watch Full VideoNo — that remaining difference is units. Manufacturers sell terabytes (1000-based); Windows reports tebibytes (1024-based) while labelling them TB. A 4 TB drive is about 3.64 TiB. Once the 2 TB slab is usable, everything you paid for is there.
Only if you made a bootable USB out of a drive larger than 2 TB, which is rare and never sensible for an installer stick. For ordinary 8–64 GB sticks the limit is unreachable — on sticks, choose by firmware (MBR for BIOS, GPT for UEFI), not by size.
Not while Windows stays on MBR: the style covers the whole disk, so the ceiling applies to all of it. Your routes are MBR2GPT in-place conversion (with the firmware switched to UEFI afterwards) or a reinstall onto GPT. Both are covered in this series.