While this is a backup of a Windows Server 2008 R2, for the purposes of this question it is Windows 7 functionality, which is why I decided to ask it on Superuser instead of Serverfault.
We backup our server to a USB drive with Robocopy. This fits our needs extremely well. Normally a backup of the changes since the last takes up to 4 hours, 2 being the norm. For the Friday backup I sometimes as a clean or formatted backup disk and this takes anywhere between 12 and 18 hours. So far so good.
Sometimes a disk will suddenly take 4 days for a full backup or well into the night for a changes-only (i.e. differential) one. The latter is not a practical problem, the former is. In both cases however the normal 70MB/s speed has dropped to 10-20 MB/s. The dataload isn't different nor is the dataset, it doesn't matter if any heavy task or heavy reading/writing is going on, it happens to different disks at different times. I always used the same USB port but changing it didn't matter. I checked temperatures in the (air-conditioned) room, no difference (it worked well in the 2 or 3 instances the airco malfunctioned and it was quite hot in there).
I can see no cause so far. Suggestions anyone?