We have not had similar cases reported nor do we have a known issue with external hard drives, "via USB, Firewire". In view of this, and based on the tests done by you so far, I'd recommend clearing the event log and loading the BIOS default settings, just to be sure:
Restart the system into BIOS by pressing <F2> during the boot process. Once in the BIOS setup utility, press <F9> to restore BIOS default settings. Reset any customized BIOS settings. Clear all DMI event logs by setting the option to "Enable", which is located in the Advanced/Event Log Configuration section of the BIOS Setup utility. Press <F10> to save the new settings and reboot the system.
If the problem continues, please, confirm what was the order (steps) you followed to install the Operating System? Please, include any BIOS settings you might have changed prior the installation. Also, please, include the Altered Assembly Number (click here) of your board.