HDD does not show up in Explorer is still accessable

Seagate Barracuda 7200.10

There is a jumper block on the back of the drive next to the power and data connections. By default, a jumper is set to limit the drive to SATA-I speeds.

When a new disk fails to show up in Windows Explorer, there can be two reasons :

1. A missing disk driver

In some rare cases, the new disk requires a proprietary driver and cannot work with the one supplied by Windows.

To check, open up Device Manager and check whether the new disks are visible under the Disk drives branch or under Unknown devices.

In the first case, Windows has correctly identified the disk. In the second case, a proprietary driver needs to be installed from either a CD supplied with the disk or downloaded from the manufacturer's website.

Once the disk is correctly identified in the Device Manager, we continue to the second phase.

2. The disk was factory-formatted in an incompatible manner

Windows is pretty particular about the way the disk should be formatted. If an external disk is partitioned into multiple partitions, it risks to be unrecognized, or worse, only the first partition is recognized. A new disk should always be examined for such problems before being used.

Open Computer Management and click on Disk Management, then wait for the disk to show up.

If the disk is partitioned, right-click all partitions and choose Delete.

Once the entire disk is one unallocated space, right-click and choose New Simple Volume, then follow the wizard. Be careful to format all disks larger than 2 TB as GPT, since the other option, MBR, will not use more than the first 2.2 TB of the disk.

Once the disk is formatted and assigned a disk-letter, it is available as a normal disk.

For more information, see this How-To Geek article :
Understanding Hard Drive Partitioning with Disk Management.