Originally Posted by TTL_2
Is there any way I can tell the System "Its ok to loose the data on the USB drive, but please sync all internal hard disks and shut down"?
Yes. It is called "Magic System Request".
It is triggered by Alt + PrintScreen/SysReq + 'another key' keys combination.
To use it, you need an enabled kernel (CONFIG_MAGIC_SYSRQ directive).
If you have a file called '/proc/sys/kernel/sysrq' then your kernel have it.
To enable the magic functions, you need a "1" in that file (echo "1" > /proc/sys/kernel/sysrq).
The 'another key' I mentioned above is one single letter, each one has a specific function. For instance, the 'm' key will dump the memory to console; 't' will dump the task list to console; 'b' will reboot your system without any sync or umounting; 'h' will print all options.
To sync all filesystems use 's' (Alt+SysReq+s).