I have and internal Seagate ST3500418AS ATA HDD and it reads this when I try to open it in windows:
E:\ is not accessible.
The request could not be performed because of an I/O device error.
Data on the disk is not that valuable but would like to be able to keep the data.
The drive may have failed, in which case some or all of the data on it will either be corrupt, or will be inaccessible to all except a data recovery company. You can test it to see whether or not is has failed. Use Seagate "SeaTools for Windows" to test it: http://www.seagate.com/support/downloads/seatools/
If SeaTools can't access the drive either, you will have to assume that it has failed.