Hi,
I'm afraid the hard disk is not really working at all - and this is why it can't be initialised and Hard Disk Sentinel also can't access it, can't detect or perform any test on it.
In order to access and use, the hard disk needs to spin up, accept commands and so. If this is not true, then it is normal that Windows can't initialise it, can't show any real usable area. Then Hard Disk Sentinel also can't detect its identification information (model ID, serial number and so) or its health. The capacity is usually 0 bytes in this case which means that there is no user-accessible data area. This is why the Disk menu -> Surface test function does not list the device too.
I'd try to connect the hard disk differently. For example if an USB docking station used - then without it, by connecting directly to SATA port.
Or vice versa: if connected to SATA port, try connecting with USB adapter / docking station.
If this is a 2.5" external hard disk, connected to single USB port, then there is good chance that the hard disk does not receive enough power for normal operation as single USB cables may not provide enough power for the hard disk drives.
This is a very common problem - and causing that the hard disk can't operate properly.
See
http://www.hdsentinel.com/hard_disk_case_spin_retry.php for more information.
But if the drive can't be detected / accessed in all cases, then I'm afraid it is completely failed - and in this case (of course) no software can help, no software can replaced failed hardware parts or make it working again.
