Are you using a 3.5" HDD? Or the smaller one 2.5" size?
if problem is HARDWARE:
Normally the problem you mentioned, it is because the H8 power supply probably cannot handle the power to your HDD + H8 together, in this case maybe you need an external power supply to your HDD, if you have an "Y" USB cable it may solve your problem.
H8 power supply is only 2A maximum current, you also can try another power supply with 3A for example, to avoid a second external power supply, however the H8 connector is small and shall be difficult to find one power supply with more current that is ready to use.
If problem is SOFTWARE:
Try to find your HDD manually, inside Kodi.
In Kodi: Go to VIDEOS -> Files -> Add Videos.. -> Browse -> Root Filesystem -> mnt -> usb_storage -> USB_DISK4 -> [Your HDD shall be located here]
Select your HDD and click "OK", now it shall be available to select the files from your HDD
I don't know why Kodi is so complicated to find a Hard disk!