As Kari mentions, any kind of continuous vibration is not a good thing. Laptops that are used onstage often have problems from bass frequencies causing the heads to bounce, which cause the OS/drive to re-read the data. At some point, if there are enough read errors the system will usually freeze. Under normal listening levels a 15-watt subwoofer isn't going to rattle your drives... I have a small recording studio with a 100-watt Yamaha sub plus two sets of monitors... we've never damaged drives but then again, the PC is in a vented sound proof enclosure and all external drives are sitting on heavy studio furniture so vibration isn't an issue.
I wouldn't set the satellite on the external, it does have a weak magnetic field, why play with fire? :-)