If you are still having the problem detecting the 2nd external hard drive, then I would like to share with you that I have also experienced the same problem when I tried to connect 3 external WD MYBOOK ESSENTIAL 1TB each, and each of the HDs with their own power supply.
I would like to also share with you, if you are still having the problem, that I have been able to connect and my Windows 7 ULTIMATE has been able to detect all 3 external HDs and each drive has its own letter assigned to it.
Each external drive (ED) is accessible like if nothing ever happened.
The issue seems to be a conflict of IDs assigned to each (ED) when we connect an (ED), then when we connect a 2nd (ED), then the 2nd (ED) is trying to use an ID that has already been assigned to the 1st (ED), therefore, the 2nd (ED) stays “offline” to avoid a conflict with the 1st (ED).
The technical term is “The disk is offline because it has a signature collision with another disk that is online”
To make the 2nd and 3rd, and 4th (ED) to be online, then you have to go to:
HOW TO FIX it:
- Computer Management
- Disk Management
- Scroll down so you can see your external drives
- You will notice that the 1st (ED) is “Online”
- You will notice that the other (EDs) are “Offline”
- Right-Click on “Offline” and a shortcut menu will give the option to turn (ED) “Online”
- Wait for a few seconds and now that (ED) will become available to be used.
I hope these findings will help you guys minimize your frustrations as I had them before I found this solution.