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References to Elements in External Memory Data Structures

You should not pass or store references to elements in an external memory data structure. When the reference is used, the block that contains the element may be no longer in internal memory.
Use/pass an iterator (reference) instead.
For stxxl::vector with n pages and LRU replacement strategy it can be guaranteed that the last n references obtained using stxxl::vector operator [] or dereferencing an iterator are valid.

Thread-Safety

The I/O and block management layers are thread-safe (since release 1.1.1). The user layer data structures are not thread-safe.
I.e. you may access different STXXL data structures from concurrent threads without problems, but you should not share a data structure between threads (without implementing proper locking yourself).
This is a design choice, having the data structures thread-safe would mean a significant performance loss.