[PaceyIV]

I have the same error when I use Calibre Portable on my PC on my NAS.

calibre 3.28 Portable embedded-python: True is64bit: False
Windows-10-10.0.17134-SP0 Windows ('32bit', 'WindowsPE')
32bit process running on 64bit windows
('Windows', '10', '10.0.17134')
Python 2.7.12+
Windows: ('10', '10.0.17134', 'SP0', u'Multiprocessor Free')
Interface language: it
Successfully initialized third party plugins: DeDRM (6, 2, 1) && Find Duplicates (1, 6, 3) && Search The Internet (1, 7, 6) && Count Pages (1, 8, 2) && INMONDADORI (0, 2, 1)
Traceback (most recent call last):
File "site-packages\calibre\gui2\actions\add.py", line 285, in add_empty
File "site-packages\calibre\db\legacy.py", line 253, in import_book
File "site-packages\calibre\db\cache.py", line 1597, in add_books
File "site-packages\calibre\db\cache.py", line 63, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 1567, in create_book_entry
File "site-packages\calibre\db\cache.py", line 1287, in set_metadata
File "site-packages\calibre\db\cache.py", line 1113, in update_path
File "site-packages\calibre\db\backend.py", line 1572, in update_path
File "site-packages\calibre\db\tables.py", line 136, in set_path
File "site-packages\calibre\db\backend.py", line 843, in execute
File "c:\t\t\apsw-bdov6u\apsw-3.23.1-r1\src\cursor.c", line 236, in resetcursor
CantOpenError: CantOpenError: unable to open database file

I can modify database, setting for example custom column on existing record, but when I try to add new books I get this error. If I close Calibre and open it again I can see the book I've created.
If I import a book from file I get the same error. If I close Calibre and open it again I can see the book I've created but without the file.
If I copy the whole Calibre Portable directory into my disk I have no error, but I've always use Calibre Portable on that network disk without problem.