There seem to be plenty of possible causes. :/
I have now copied a part of the library to the laptop and used the library maintenance - restore option.
Worked well except this error message:
Spoiler:
calibre, version 1.33.0
WARNING: Success: Restoring the database succeeded with some warnings. Click "Show details" to see the details. The old database was saved as: <snip>
Failed to restore the books in the following folders:
C:\Calibre Portable\Calibre Library\Andrew Lane\Young Sherlock Holmes- Death Cloud (19078) with error:
Traceback (most recent call last):
File "site-packages\calibre\db\restore.py", line 253, in restore_books
File "site-packages\calibre\db\cache.py", line 57, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 1787, in restore_book
File "site-packages\calibre\db\backend.py", line 1668, in restore_book
File "site-packages\calibre\db\backend.py", line 803, in executemany
File "c:\cygwin\home\kovid\sw\build\apsw-3.7.17-r1\src\cursor.c", line 231, in resetcursor
ConstraintError: ConstraintError: columns book, format are not unique
Is it just a damaged (?) db entry that causes a problem here or something more severe?
It is Calibre Portable btw.
OS is Win7, 64bit.
P.S. No problems or crashes or anything and Calibre shows up fast when opened.
Will have to see what happens with increasing memory load and runtime but it already is much better than it ever was on the old system.