With seek changed the error. Now I have:
RuntimeError: [enforce fail at ..\caffe2\serialize\inline_container.cc:145] . PytorchStreamReader failed reading zip archive: invalid header or archive is corrupted

It kind makes sense of this error, I only concatenated some buffers. I don’t think that it is a well built structure for torch to load, isn’t it? The files are working independently, this isn’t a file divided in multiple files for some reason.
Let’s say I have a.pt and b.pt, inside a.pt I have a dataset, inside b.pt I have another that I have to merge together. If I run torch.load('a.pt') it works, same thing for b.pt.