I am using the following script to download a site context at one before I do
the next one:

 date
 httrack  \
  --extra-log  \
  --debug-log  \
  --verbose  \
  --extended-parsing=N  \
  --near  \
  --test  \
  -U \
  --user-agent "${_USR}"  \
  --robots=0   \
  "${_START_URL}"  \
  "+mime:text/html +mime:application/pdf"   \
  "-r6" >> "${_LOG}" 2>&1

 However I did notice hhtrack was deleting what it had previously
downloaded!!! Why is that? How do you instruct hhtrack not to delete local
data once downloaded?
  This is what I see in the log files after hhtrack deletes the files

06:54:21	Info: 	Purging www.nysedregents.org/Physics/616/phys62016-exam.pdf
06:54:21	Info: 	Purging www.nysedregents.org/images/pdf-icon.11.delayed
06:54:21	Info: 	Purging
www.nysedregents.org/Physics/616/phys62016-ansbklt.pdf
06:54:21	Info: 	Purging www.nysedregents.org/Physics/616/phys62016-rg.pdf
06:54:21	Info: 	Purging www.nysedregents.org/Physics/616/phys62016-cc.pdf
06:54:21	Info: 	Purging www.nysedregents.org/Physics/615/phys62015-exam.pdf
06:54:21	Info: 	Purging
www.nysedregents.org/Physics/615/phys62015-ansbklt.pdf
06:54:21	Info: 	Purging www.nysedregents.org/Physics/615/phys62015-rg.pdf

...
 lbrtchx