External HD connected to 2 PC's

yeah you need a NAS device

No, you cannot connect two computers to two ports on an external drive and expect them both to control the drive. These units really are not designed to deal with clashes of use by two devices. On the other hand, you could set one up connected to only one device at a time and physically go to it and change which user cord is plugged in. But it is my understanding you want this backup-and-forward system to operate without human intervention.

Obviously a simple NAS is not your solution because you cannot attach the backup server to the network containing the NAS. If I understand your system, the backup server is configured to be an exact clone of the live server. To use it, someone has to physically come in and disconnect the live server from your network and then connect in its place the backup server.

I have no idea whether the components for this are necessary or whether it could work, but think about this. You need a disk backup device to which you can connect your live server via a network connection. Then you need a non-network connection system to make a connection from that device to the backup server, such that the backup server can access and download any file(s) the disk backup device has. Maybe an NAS disk backup device with a serial or USB port connection to a console that controls it? Or, a computer serving as the disk backup system, with a direct serial (or other) connection to the backup server that allows file transfers on command from the backup server unit. How about using a fast IEEE 1394 (Firewire) communication link between disk backup and backup server units?

Not being a network administrator, I'll ask another question. For this situation in which two servers are configured exactly the same, is it sufficient to put them on separate segments of a segmented network? That is, if you had a central small computer with two LAN controllers establishing two main segments of the network, can the two servers be connected to different segments and coexist? If so, you could do that and attach an NAS to one segment, then access it from both.

This might be a solution:

Use a NAS-type external hard drive with an ethernet A-B switch between the drive and the 2 servers.
For NAS, remote controllable ethernet A-B switches:
See:
http://www.l-com.com/productfamily.aspx?id=3100

OR

Use a USB 2.0 external hard drive with a USB A-B switch between the drive and the 2 servers.
For USB, remote controllable USB A-B switches:
See:
http://www.electrostandards.com/Pr [...] =10+82+390