Unfortunatelly virtualized os does not have access to usb disk files, so you cannot store the logs directly to external flash.
I already was asking here on the forum more than year ago the same question: http://forum.mikrotik.com/t/remote-logging-from-one-mikrotik-to-another-mikrotik/70456/1
I also argumented that syslog server would be much more useful than ip cloud. http://forum.mikrotik.com/t/ip-cloud/77987/13 No answer to this.
You should search the forum before you asked.
Anyway, now I do not think it is necessary when running dude on pc. Unfortunatelly it needs running pc. I hope I will deploy some small mipsbe devices with 5.x ros and try to run dude on them. Hope it will work. In this case I would be able to get rid of dude from pc.
Have a look at dude, it has quite good syslog server functionalities.