Nils I wish I still had the problem (mmmm... maybe not... haahaha), but since I had it I split the file and now things seem to be working just fine. Nevertheless, I can try to replicate it to help you debug it. Are you interested? Harold I suspect it was failing because: On the Devices tab, the entry for my brother's computer said that the status was "[up-arrow] 100GB" and never actually decreased the number during the 24h we were trying to sync the file. On the Transfers tab, the entry for the folder uploading to my brother's computer showed that the upload speed was going from 0 to 80kB/s every 20-30 seconds. After that ammount of time, it usually stopped (0kB/s) and went up again. That kept repeating for the whole 24h.Once we split the file, the Devices tab number started decreasing and the Transfers tab shows a "constant upload speed" of 60-80kB/s.