Errors while adding: failed to add any books

Have been using Calibre with great success, never a problem, for several years. A problem just surfaced:

calibre, version 2.56.0
ERROR: Errors while adding: Failed to add any books, click "Show details" for more information


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Failed to add the book: The Iron Horse
C:\Users\Hope\Desktop\Books\Marston, Edward-The Iron Horse (The Railway Detective #4).epub
With error:
Traceback (most recent call last):
File "site-packages\calibre\gui2\add.py", line 377, in add_book
File "site-packages\calibre\db\legacy.py", line 214, in create_book_entry
File "site-packages\calibre\db\cache.py", line 57, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 1520, in create_book_entry
File "site-packages\calibre\db\backend.py", line 809, in execute
File "c:\cygwin64\home\kovid\sw\build\apsw-3.8.2-r1\src\cursor.c", line 231, in resetcursor
BusyError: BusyError: database is locked


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Failed to add the book: Murder on the Brighton Express
C:\Users\Hope\Desktop\Books\Marston, Edward-Murder on the Brighton Express (The Railway Detective #5).epub
With error:
Traceback (most recent call last):
File "site-packages\calibre\gui2\add.py", line 377, in add_book
File "site-packages\calibre\db\legacy.py", line 214, in create_book_entry
File "site-packages\calibre\db\cache.py", line 57, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 1520, in create_book_entry
File "site-packages\calibre\db\backend.py", line 809, in execute
File "c:\cygwin64\home\kovid\sw\build\apsw-3.8.2-r1\src\cursor.c", line 231, in resetcursor
BusyError: BusyError: database is locked


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Failed to add the book: The King's Evil
C:\Users\Hope\Desktop\Books\Marston, Edward-The King's Evil (Christopher Redmayne, #1) .epub
With error:
Traceback (most recent call last):
File "site-packages\calibre\gui2\add.py", line 377, in add_book
File "site-packages\calibre\db\legacy.py", line 214, in create_book_entry
File "site-packages\calibre\db\cache.py", line 57, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 1520, in create_book_entry
File "site-packages\calibre\db\backend.py", line 809, in execute
File "c:\cygwin64\home\kovid\sw\build\apsw-3.8.2-r1\src\cursor.c", line 231, in resetcursor
BusyError: BusyError: database is locked


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Failed to add the book: The Repentant Rake
C:\Users\Hope\Desktop\Books\Marston, Edward-The Repentant Rake (Christopher Redmayne, #3) .epub
With error:
Traceback (most recent call last):
File "site-packages\calibre\gui2\add.py", line 377, in add_book
File "site-packages\calibre\db\legacy.py", line 214, in create_book_entry
File "site-packages\calibre\db\cache.py", line 57, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 1520, in create_book_entry
File "site-packages\calibre\db\backend.py", line 809, in execute
File "c:\cygwin64\home\kovid\sw\build\apsw-3.8.2-r1\src\cursor.c", line 231, in resetcursor

I'm using Windows 7 Professional on a Dell Precision 490, Intel Xeon CPU 5160@3.00GHz, 16GB Ram, 64-bit OS.

I am using a SanDisk Solid State for my programs and a standard disk to store the Calibre directory. I'm backing up the library on an external drive through USB port. What is happening, and how can I, or CAN I, continue to add books to my library?