SATA hard drive controllers require a feature called port multiplier to read more than one hard drive per SATA or eSATA port. The port multiplier feature is not standard on many SATA hard drive controllers.
If your computer is only recognizing one of your hard drives, you need to plug the eSATA cable into a SATA controller that includes the port multiplier feature. If none of your current hard drive controllers include this feature, you can add a hard drive controller that does support port multiplier in any available expansion slot (for example, PCI, PCI-Express, PCMCIA, CardBus, or ExpressCard). StarTech.com has hard drive controllers available that support the port multiplier feature. To view the hard drive controllers, click here: http://startech.com/Cards-Adapters/HDD-Controllers/SATA-Cards/?filter_PORTMULTIPLIER=Yes.
If you have a hard drive controller card that does support the port multiplier feature but it is only reading one hard drive at a time, update the drivers of the hard drive controller. To find the most current StarTech.com drivers, click here: http://www.startech.com/Support.