Eva,
Let's go step by step...
First, you have a big library (media files). Just to compare, your need twice the time I need to do the same copy to an external drive from the Explorer (about 500GB for 70 000 media files). So, it's important to choose an efficient (and safe) method. Your catalog folder is by default stored on the C: drive. You find its location and size from the menu Help >> System information. The catalog folder can be stored (copied or moved) anywhere else (it is shown in 'custom' location in the 'manage catalogs' dialog list); its contents are not changed.
You understand that the database in the catalog stores the information for drive and path of each item of you library and that the catalog is never aware of changes you are doing outside of the organizer.
If you use the explorer to move one photo file from you main drive to the external drive, the recording of the drive and path of that item is not updated. The organizer can't find any file at the old location; it shows the file as 'missing' or 'disconnected'. The other pieces of information in the record of the database are still unchanged, exif, tags, thumbnail. You have to tell the organizer to 'reconnect' that is to find the new location and to update the record with the new drive/path. If you know the new location, you can guide the organizer to it and the file will be reconnected. If you don't know, the organizer will start a search comparing the data in the catalog to the data of all other files present on your computer. Just do the test for a single moved photo file and you'll see how long scanning all your files will be...
If all your library has been moved to a new drive, or even if a folder has been moved, doing the scan for each disconnected file will be much too long if you don't help the search in the 'reconnect' dialog by guiding the scan to the new folder.
Photoshop Elements (PSE) knowledge base.
In your present situation, reconnecting is totally unrealistic. It would mean making all your files 'disconnected' on the master drive first, by deleting them or making them appear as such; for instance in renaming temporarily the master folder. Then the reconnect function will try to reconnect... and that will be too long even if you guide the scan for the new drive.
Now, what if you use the options of the organizer to move one file or a folder, or even the whole master folder of your library to the external drive? You set the left folders panel to tree view and you drag and drop the master folder to the desired drive. That's all. The organizer uses the explorer routines to move the files to the new location and at the same time, it updates the contents of the catalog. The move by itself takes the same time as your move from the explorer, and the re-indexing adds some time (negligible in fact). Nothing to reconnect. Your files are really 'moved', not 'copied', your master drive is clean. The process can be global, or folder branch by folder branch. The process is really safe, even if you have a problem in the move: I had once a power failure... Nothing lost, the files are not really deleted until the transaction is finished and successful. And you have your safety copy on another external drive. Just let the process run by night; there is no progress bar but the task runs ok even if the task manager says the organizer is not responding.
Now, let's have a look at the backup and restore process:
Photoshop Elements (PSE) knowledge base.
It's the safest process. It does take the same time as your copy process plus the move described above. After the restore, you have the restored folder tree on the external drive, plus a backup allowing restore on any other drive or computer with the links updated. You still have the original folder tree in your main drive. You delete it manually from the explorer when you are ready.
Just note that you have now a new catalog, also located on the external drive. If you prefer it on the default internal drive, the catalog manager has a 'move' button to move it back to default location.
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