I had bad times every time i tried to use nbackup to do what you want to do (frequent backups of online databases, specially when there are many active connections).

I tried for 3 or 4 times in production databases, with different firebird versions, hardware and OS, and every time i had my databases hardly corrupted after some days, so i don´t think it is a coincidence...

Today i don´t use nbackup anymore and don´t plan to use it further, but i solved my backup needs another way.

I do a regular backup of my database using gbak, and sync the fbk file with rsync.

You will be surprised with the small size of the delta file between two fbk files taken in distinct times, so it is a good candidate to be used with rsync, instead of the main database file.

If you decide to use nbackup every 5 minutes and don´t have issues after some time, please let me know, maybe it is more reliable now.

Regards,

Fabiano