ck read failed

[alinush]

Describe the bug

alin@alin-a01 [~/] (master) $ ck read BMMV19
Added 'q' tag
Added 'u' tag
Added 'e' tag
Unexpected error while tagging BMMV19 with '/
Traceback (malinost recent call last):
  File "/Users/alin/repos/ck/citationkeys/tags.py", line 186, in tag_paper
    os.symlink(os.path.join(ck_bib_dir, pdfname), os.path.join(pdf_tag_dir, pdfname))
OSError: [Errno 30] Read-only file system: '/Users/alin/Dropbox/Papers/BMMV19.pdf' -> '/BMMV19.pdf'
Traceback (most recent call last):
  File "/Users/alin/repos/ck/ck", line 1164, in <module>
    ck(obj={})
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/alin/repos/ck/ck", line 444, in ck_read_cmd
    ctx.invoke(ck_tag_cmd, silent=True, citation_key=citation_key, tags="queue/reading")
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/alin/repos/ck/ck", line 646, in ck_tag_cmd
    if tag_paper(ck_tag_dir, ck_bib_dir, citation_key, tag):
  File "/Users/alin/repos/ck/citationkeys/tags.py", line 186, in tag_paper
    os.symlink(os.path.join(ck_bib_dir, pdfname), os.path.join(pdf_tag_dir, pdfname))
OSError: [Errno 30] Read-only file system: '/Users/alin/Dropbox/Papers/BMMV19.pdf' -> '/BMMV19.pdf'

To Reproduce
See log above.

Desktop (please complete the following information):

• OS: OS X Catalina

Additional context
Paper was tagged with #queue/to-read