nnn,
Colinc is right on. Practically when booting from CD or USB in DOS that device will be assigned as a:, the next drive that DOS recognizes (Fat drive) will be assigned c:. If there are no other valid DOS devices the the ram drive will be assigned c:. So, if you boot from a USB key saving the logs to a: will work. If you boot from CD with a USB Key inserted or have a hard drive with a FAT partition on it they are usually assigned c: with the ram drive being moved up to d:.
It takes a little getting used to and your point about it not being obvious is well taken, we will look into rectifying this in the future.
-Fred