I had a download at 54% of around 700 Meg.
In this case there was a crash.
After a reboot the % to go was very low.( 5%)
I also deliberately exited another time and the same
thing happened, quite a substatacial loss of data.
The deliberate exit was very unclean, by the way.
Hanging processes blue screen ...
A clean deliberate shutdown option would be nice.

Anyway I was interested in the frequency of flushing to disk.
Also is if you are downloading a 200Meg file and you have
512 Meg how likely is it to buffer the majority of the file
in RAM?

It at least appeared to work this way for me.

If you go into the temp directory you can alt+enter (properties) a file and see it go up all the time, so I don't think it writes straight to disk.

What probably happened was you got a BSOD as XoloX was writing resumeinfo.txt to disk (whch it does at the start and end of each segment download), and the segments with the largest chunk of the file you were downloading hadn't been written to the file yet. All files in the /temp directory not listed in the resumeinfo.txt file are presumed bad or unneccesary and are deleted.

As for the clean exit... I'm not sure, probably due to XoloX not being able to interpet the corrupt resumeinfo file correctly. Deleting resumeinfo.txt will probably fix it, though you will loose all yoru current download jobs. I have BSOD'd, alt+ctrl+delete killed, and power-off'ed to kill XoloX and have not had any problems yet resuming. I must just be lucky it's has never been writing resumeinfo.txt at the time.

As a side note/suggestion, perhaps XoloX should rename resumeinfo.txt to resumeinfo.bak before updating? at least then a user that suffered a serious crash could use the backup copy instead.