I am presently running FCP-X with a 3TB external HD for library and backups. Eventually all HD's will fail. Therefore, if I purchase another 3TB HD and transfer all files from one HD to the other will the second HD act as though it were the original HD?
Aside from a new name, yes, if it is not a system drive, it will act the same.
But, I'd recommend getting something like Carbon Copy Cloner and do daily backups with it. The original backup will duplicate everything. After that, it only backups what has changed, so the backups don't take very long at all. This will be an exact duplicate and act the same if your working drive every fails. It also does check-sum and other things to verify the drives are working properly and the data transfers are in tact. Tasks that aren't done with regular drag-and-drop duplicates. Also, you can schedule the backups to happen in the middle of the night while you're asleep.
Nemo wrote: ... running FCP-X with a 3TB external HD for library and backups...if I purchase another 3TB HD and transfer all files from one HD to the other will the second HD act as though it were the original HD?....
As FCPX.guru said, yes except for the drive name. However if your drive name is different, certain aspects of FCPX will not work the same.
E.g, if using external media (not media copied within the library) it will require relinking. The drive name or drive ID is part of the path to the media files and that is stored within the library. If the drive name changes, the only way to change the drive name or ID within the library is relink. This usually works well.
If using external proxies (vs normal proxies which are inside the library), those cannot be relinked and they must all be rebuilt.
If you maintain the same drive name, everything related to FCPX should work the same.