I just tested it again and Alan is right, Adam must be tired.

It WILL overwrite the fbk file, but expects to have to create the log
file (if you use -y). It FAILS if the log file exists. It is like
fmCreate is used but not fmOpenWrite.

I would consider this inconsistent behaviour. IMO, either you allow
overwrite both files, or you refuse to overwrite both files, or
possibly add a flag to gbak to tell it it can overwrite it (sort of a -
R equivalent), but I am sure Helen will be all for that one ;)

My sleepy advice still stands though, make sure you do not overwrite
your only backup (copy it elsewhere as soon as gbak finishes)