Excerpts from Pedro Emílio Machado de Brito's message of Mon Jun 10 02:53:22 +0200 2013:
I've been having this sort of problems with removable storage lately (copying multiple GBs of songs in a few seconds, except not really). The workaround I found out is to run the sync command after copying. That could take some time as all your data is being actually written to disk.
I believe this is related to the write cache, please let me know if you find a better solution to this.
Forgot to mention sync. I ended equally ultra fast.