The export utility creates a logical backup of your database, not a physical backup. Logical backups have many downsides. One, you can only recover up to the point of time the export dump was created. You cannot roll forward any changes. Two, recovery from a logical backup is a manual process, which can be filled with human errors. Three, when you import a table from your export dump, internally in Oracle, this table is a new table even though it may have the same name.
There are two options in Oracle 7 to take a backup: hot and cold backups. Cold backups are the easiest to implement. Simply shut down the database and use OS utilities to copy your database datafiles and control files. Hot backups are more difficult. Basically, you place the tablespace in backup mode and then copy the tablespace's datafiles. Once done with that tablespace, place it back in normal mode and proceed to the next tablespace. All of this is covered in the Backup & Recovery Guide documentation for your version.
I would seriously consider upgrading the Oracle 7.3 database to Oracle 9i or 10g. In those versions, you can use Oracle's Recovery Manager (RMAN) to perform your backup and recovery.
I also recommend purchasing a copy of Oracle Backup and Recovery Handbook by Velpuri and Adkoli on Oracle Press. It goes through many different backup and recovery scenarios.
Dig Deeper on Oracle database backup and recovery
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