> > 1) Use the gbak from FB 1.5 to create the backup file.
>
> step 1:
> Tried this approach:
>
> ISC ERROR CODE:335544379
>
> ISC ERROR MESSAGE:
> Unsupported on-disk structure for file D:\DATA.GDB; found 32779,
> support 10

You were using the 1.5.3 gbak client. Were you connecting to the 2.0
engine?

> > 2) Use a datapump tool (eg clevercomponents IBDatapump)
>
> We have databases of 150 GB a data pump just doesn't work here

I don't understand, what is the data pump doing that gbak is not?

> Shouldn't all backup files be compatible with each other?

Should an old version of the engine understand new features?

> Definitly the transportable ones should be.
>

Not sure whether transportable means this, or whether is takes into
account endian differences between different machines. Someone with
more knowledge may need to help you there, but from what I understand,
the 1.5.3 gbak SHOULD be able to back up a FB 2 structure providing
you have not been using new features the old engine just wont understand.

Adam