File > Client File Maintenance > Restore > navigate to files on external hard drive.  Aren't you using the C drive to store the files though and using the external drive as one of your backups?

If you used the backup function in PS to backup to an external hard drive you will use the restore function in PS. If you just copied the data file to an external drive it will be more involved, as I believe you will have to use "open client" for each client, then navigate to the external hard drive to open each client.