Você não pode fazer fdisk porque você já está com o dobro do máximo da MBR com um dispositivo de 4TB em setores de 512b. Você precisa formatá-lo com o GPT.
De wikipedia :
The organization of the partition table in the MBR limits the maximum addressable storage space of a disk to 2 TB (232 × 512 bytes).
Pegue o pacote gdisk e reformate o disco (embora pareça que já deveria estar) . Se você estiver olhando para esse disco a partir de uma máquina que não pode montar um disco GPT, o MBR de proteção explicaria o erro hdparm .
Senão eu vejo que o tamanho do setor físico está listado em 4kb - o que faz um pouco mais de sentido - e torna a MBR viável, mas, como você viu, é bastante difícil de manejar. A solução vai estar em uma tabela de partição atualizada.
Mais de wikipedia
Since partitioning information is stored in the MBR partition table using a beginning block address and a length, it may in theory be possible to define partitions in such a way that the allocated space for a disk with 512-byte sectors gives a total size approaching 4 TB, if all but one partition are located below the 2 TB limit and the last one is assigned as starting at or close to block 232−1 and specify the size as up to 232−1, thereby defining a partition which requires 33 rather than 32 bits for the sector address to be accessed. However, in practice, only certain LBA-48 enabled operating systems, including Linux, FreeBSD and Windows 7 that use 64-bit sector addresses internally actually support this.
Due to code space constraints and the nature of the MBR partition table to only support 32 bits, boot sectors, even if enabled to support LBA-48 rather than LBA-28, often use 32-bit calculations, unless they are specifically designed to support the full address range of LBA-48 or are intended to run on 64-bit platforms only. Any boot code or operating system using 32-bit sector addresses internally would cause addresses to wrap around accessing this partition and thereby result in serious data corruption over all partitions.
For disks that present a sector size other than 512 bytes, such as USB external drives, there are limitations as well. A sector size of 4,096 results in an eight-fold increase in the size of a partition that can be defined using MBR, allowing partitions up to 16 TiB (232 × 4096 bytes) in size. Versions of Windows more recent than Windows XP support the larger sector sizes as well as Mac OS X, and the Linux kernel has supported larger sector sizes since 2.6.31 or 2.6.32, but issues with boot loaders, partitioning tools and computer BIOS implementations present certain limitations, since they are often hard-wired to reserve only 512 bytes for sector buffers, causing memory to become overwritten for larger sector sizes. This may cause unpredictable behaviour as well, and therefore should be avoided when compatibility and standard conformity is an issue.
Há um link para um Western Digital .pdf discutindo a possibilidade de usar tamanhos de setor MBR maiores em drives muito grandes. Aparentemente, o kernel 2.6.32 foi o primeiro a introduzir suporte para esse recurso. Eu não sei se é uma opção de tempo de compilação ou não, mas talvez seja muito novo quando o seu próprio kernel NAS foi construído.
Aparentemente, existem outras possibilidades. A partir do .pdf
Older Linux Kernel version (all 2.4 and pre‐2.6.32) computing
environments with a legacy BIOS and MBR partition table scheme
encounter a barrier at 2.19 TB because they can address only up
to 232 logical blocks. Be sure to use kernels that contain support
for drives greater than 2.19TB. The kernels released after April
2010 have support for large capacity drives using 4096 sector sizes.
...
How You Can Take Advantage of Large Capacity Drives in Linux
• Use fdisk from util‐linux‐ng >= 2.17.2 or parted/gparted
• Use +size {M, G} convention to specify "Last sector"
(e.g. +5G to create 5GiB partition) then fdiskl aligns the
size to physical block boundary
• Remember that fdisk(8) always follows your wishes ‐‐ it means
that if you explicitly define first/last sector number then
the partition could be misaligned.
• Start the extended partition at sector 64 (the default is 63),
and end it at sector (total amount of sectors on the drive – 1)