Sean,

>> I did have some issue with it 1-2 years ago, with a rather large DB.
>> Some problem with transaction commit during nbackup locked state. I never
>> had time to investigate if it was really caused by nbackup, but it only
>> happened in that state...
>
> A review of the project tracker finds that there are no open issues with nbackup functionality.
>
> The closed items list finds that there were some cases closed previously
>
> Fixed in v2.0.1 and later:
> NBackup doesn't delete the delta file after successful backup on WIn32 CS (http://tracker.firebirdsql.org/browse/CORE-1139)
>
> Fixed in v2.1.2 and later:
> Nbackup database locking not working correctly (http://tracker.firebirdsql.org/browse/CORE-2266)

To quote Dmitry from a few weeks ago (around mid Jan. 2012), the usage
of nbackup in 2.1 with Classic under high load can be "problematic". It
seems that there was quite a re-write in 2.5 though.

I have no evidence in respect 2.1 though, except I know one site that
quit using nbackup with 2.1.3 and 2.1.4 Classic with ~300 attachments
due to stability, corruption problems. Without nbackup, the system is
more or less working fine now.

Regards,
Thomas