> gbak -t -v -user SYSDBA -password <password>

"C:\MyDataBases\MyDB1.FDB" "PC2:C:\MyBackups\MyDB1.fbk" gets
an error

Of course it gets an error. You are making a backup. roughly, the
syntax is:

gbak.exe [flags etc] [database connection string] [place to put the
backup file]

On which file system is "PC2:C:\MyBackups\MyDB1.fbk" going to be a
valid path and filename?

> gbak -t -v -user SYSDBA -password <password>

"P:\MyDataBases\MyDB1.FDB" "C:\MyBackups\MyDB1.fbk"
gets an error

What is 'P'?? If it is a network drive? (see below)

> gbak -t -v -user SYSDBA -password <password>

"\\PC2\C\MyDataBases\MyDB1.FDB" "C:\MyBackups\MyDB1.fbk" gets
an error

Please go back to my first message and read the first sentence. "The
Firebird database must reside on a file system under the direct
control of the Firebird engine.".

Or you could read Helen's post "On Windows there is one and only one
way that this is physically possible AND YOU MUST NOT DO IT....that is
by fooling the embedded server into believing the database is local,
by way of a network mapping. REPEAT REPEAT REPEAT !! DO NOT DO THIS
!!" Perhaps that was a little too subtle?? ;)

You can not ask a Firebird engine to connect to a database on a remote
file system, nor can you trick it by mapping a network drive.

gbak -t -v -user SYSDBA -password <password>
"PC2:C\MyDataBases\MyDB1.FDB" "C:\MyBackups\MyDB1.fbk"

would be ok, because the computer it is run on would be asking the
Firebird engine running on PC2 to open the database file as a proxy
for you.

But of course, this will not work, because you are using the embedded
server on PC2 (according to your previous post, only PC2 needed access
to this database)

"2. With MyApp.exe, a user in PC2 (the client) can access PC1DB1
(which is in the server) (at the same time the user in Pc1 and the
user in PC3 are accessing this database too), and at the same time the
user in PC2 can acess PC2DB1 (which for him is a local database). This
access to PC2DB1 can be exclusive."

Perhaps you need to rethink and restate your requirements?

Adam