4. The backup attempt eventually failed with a ___File name too
long___ error.
...
OSError: [Errno 63] File name too long: '/Volumes/Ext_BU/Users/kwh/
Assuming you were updating a backup started earlier, this is a good
sign, since for backwards compatibility it needs to keep quoting if
quoting was enabled before. However, filename too long errors should
be recoverable, and not crash rdiff-backup. Perhaps you could start
the python interpreter and type this in:
import errno
errno.ENAMETOOLONG
36
errno.errorcode[63]
'ENOSR'
(example values are what my system says---yours must be different if
it is crashing rdiff-backup).