First of all, adding the partition in lilo won't help.
Didn't think it would.
But where do you install lilo? (the boot= part in your lilo.conf). I'm thinkingperhaps lilo is installed in the boot sector of your data partition (which would be strange but one never knows) and windows then detects the partition changed or something similar and disables access to it.
MBR (Boot=/dev/sda) I think which is probably why the partition disappears i.e LILO overwrites the MBR partition info somehow.
Next time your partition disappears, try going through the windows error messages (event log) to see why it is disabling it. Also, when booting to linux, try using cfdisk to see if something might be wrong with the partitioning scheme. It could be a multitude of things I guess.
Partition disappears on reboot, whether I go into Windows or Linux, hence I suspect LILO. Nothing obvious in event viewer and qtparted shows the partition as hidden, but I can mount and access the partition in Linux, will try cfdisk.
greets, Wim
Thanks againWackojacko