managing tablespaces like files?

The server of my client/server program is currently using SQLite, an
embedded file based SQL engine. I am looking for a client/server based RDBM
and have narrowed it down to either PosgreSQL or Firebird. I would prefer
to use PosgreSQL, but the management of the physical files are concerning
me.

A very key element of the program I am developing is the ability for the
user to pickup one folder that contains all the data for that "document" and
copy it somewhere else, then allow the program to read it from that other
location. It is equally important that when the database server initially
start, if one of the "tablespaces" was deleted by the user, the database be
none the wiser.

This is, of course, not a problem with Firebird since it is file based.
What I could do is simply disconnect from the tablespace each and every
time the server program shutdown and reconnect to that tablespace each time
the server starts up, pointing to a new tablespace if need be. So how would
one:

1: "disconnect" from a tablespace, in such a way that the tablespace, nor
it's contents is changed.
2: "connect" to an existing tablespace.

Sam