How to create a second partition on an external disk and is it safe?
Yes you can repartition without losing data. Using Disk Utility, perform a repair on your drive to make sure the drive is free of errors (even better, use Diskwarrior if you have a copy). Then unmount your drive but don't eject it. Select the drive in the left hand pane, then go to the Partition tab. On the Partition Layout section click on the "+" to create a new partition. Optionally you can specify the new partition size. When you're happy hit the Apply button and wait.

If you've absolutely critical data on the drive make sure you have a backup (which you should have anyway) but I've done this many times without an issue.