New WD Black 6TB in old PC

The BIOS of your motherboard does not support hard drives larger than 2 TB. The hard drive will thus not correctly recognized by the BIOS. But Windows can. Therefore, it is not important whether the BIOS recognizes the hard drive properly.

Very important here is the installed SATA controller drivers under Windows. Older proprietary drivers have a 32-bit LBA limit (maximum 2.2 TB). So also the Intel AHCI driver from the Matrix Storage Manager Series (MSM). The MSM driver has never supports hard drives larger than 2 TB and should not be installed. Use either the standard AHCI driver from Microsoft or an Intel AHCI driver from the Rapid Storage Technology Series version 10.1 (and higher). The RST driver supports the chipset of your motherboard up to version 11.2. Newer RST driver can no longer install. I prefer the standard AHCI driver (if no RAID is used).

Use for setting up the hard disk, the Disk Management of Windows. No other tool. If the hard drive is not recognized correctly in the Disk Management tool (1493 GIB), the installed driver is not suitable. If the hard drive is correctly recognized, GPT must be chosen as partition style. Only then will the full capacity can be utilized.

/Edit
I assume that you use at least Windows Vista as the operating system.