One possible issue with USB devices is common ground causing ground loops (both DC and earth).

Low cost USB isolators are now available from a number of sources. We have tested the isolator from Olimex and it works fine with no issues.

When using it an isolated +5V supply is required for KFLOP

Mouser has them in stock for $41 qty 1. They are also available in Europe.

Group: DynoMotion	Message: 4303	From: TKSOFT	Date: 3/24/2012
Subject: USB Ground Isolation