files do not seem to like me :-(

Gordon,

not the file size is the problem, but the limited number of append operations (provided that my theory is correct!)

I would perhaps have to write, let's say 1260(a few bytes), e.g. 12*60*100 = 72kB - but in 12*60 = 720 steps.

This can not be done incrementally, can it? So I will have to preallocate room for the longest sequence I expect and keep track of what I've already written myself - carefully avoiding writing to offset 0 as that creates a new file (well, it wastes 1 byte, but that's ok)