I have a mystery regarding pauses after file copies on windows server 2088 (and other releases)

When copying large files, like vhds, to locally attached USB disks I often see a long pause after the copy has completed 100%.

As an example: robocopying vhd files. The bytes read/written count matches the vhd file size and robocopy shows 100% but it pauses for several minutes. If I do nothing it will continue, but I will have to wait for quite some time - about the same amount of time as it took to get to 100%. The bytes read/bytes written counters for robocopy do not change.

My first thought was that the AV had to scan it, but I'm looking at a machine right now which doesn't have an AV installed and this is occurring, so impossible. No other processes are showing read/write byte counts as going up.

The behavior is the same if I use the copy command or xcopy.

I've seen this on other systems but have never worked out what the cause is. Anyone got any suggestions as to what might be going on?