I'm performing a first backup of a modestly sized archive, of ~3TB of ~10,000 medium-sized files. This is going reasonably slowly (certainly far slower than my upload speed), but that's fine. The backup, however, is semi-regularly going backwards. I've kept a track of both the amount of data and the number of files uploaded, and somehow they both go down by a large chunk on a near-daily basis.

The backup process is running continuously, my internet connection is rock solid, the computer running the backup is doing nearly nothing else, and the files being backed up aren't changing. I was hoping someone here might have some insight as to why this might be happening. I understand that there might be the odd parity check which may invalidate a few uploaded files, but we're talking up to 10% of the completed files disappearing.

I haven't been able to pin this down to a particular time of day, or indeed find any pattern in when or why this occurs at all. My network monitoring tools suggest there's no interruption, so I'm at a bit of a loss as to why this is happening. I don't mind the backup being slow, and I'm quite happy to be patient, but at the moment the net gain of files uploaded seems to be a few gigabytes a day at most. Has anyone encountered this, and might there be something I'm missing? A cursory Google search is extremely light on the matter. Any help much appreciated.