I set my SecDataDir setting as suggested in the modsecurity handbook:
SecDataDir /opt/modsecurity/var/data/

then I added a rule:
SecAction "phase:1,t:none,log,pass,initcol:RESOURCE=%{REQUEST_FILENAME}"

The debug log says:

[26/Jul/2011:12:03:07 +0100] [localhost/sid#b7f58848][rid#b6ef1058][/][9] Resolved macro %{REQUEST_FILENAME} to: /
[26/Jul/2011:12:03:07 +0100] [localhost/sid#b7f58848][rid#b6ef1058][/][4] Creating collection (name "RESOURCE", key "/").
[26/Jul/2011:12:03:07 +0100] [localhost/sid#b7f58848][rid#b6ef1058][/][9] Recorded original collection variable: RESOURCE.UPDATE_COUNTER = "0"
[26/Jul/2011:12:03:07 +0100] [localhost/sid#b7f58848][rid#b6ef1058][/][4] Added collection "RESOURCE" to the list.

but the /opt/modsecurity/var/data/ directory is completely empty. Why don't I see the RESOURCE collection in /opt/modsecurity/var/data/ ?

regards,
Chris