My guess that windows is in a loop of reading bytes from the source and writing the bytes to the destination. As munin mentions, the appropriate transaction siz

My guess that windows is in a loop of reading bytes from the source and writing the bytes to the destination. As munin mentions, the appropriate transaction size of writes to a device will be related to how much data the CacheManager chooses to allow to be buffered and not flushed to the actual device. Which is huge for a hard disk and very small for a usb drive since windows expects it might be pulled from the port at any point. Now, consider that calling read() on a socket that's really an SMB share is a complicated, multi-round trip action. The read must involve some locking in case someone else tries to write to the file in between, and probably a ton of other stuff. In my experience, even doing something like opening a 5k php file in vim from a samba mount can take a few seconds. That overhead is getting paid far more often in the network -> USB case than it is in the network to HD case.