Storing data on a external drive used for backup
Time Machine will only have access to its own backups.  So, anything besides Backups.backupdb will not be touched.

Keep in mind though that Time Machine is set to use up the entire drive eventually.  Having it shared with outside data may have adverse effects for either storing more of your data outside Backups.backupdb or for Time Machine (depending on which part fills up the remaining space first).

While it is safe to use a Time Machine-marked drive with other data, it is recommended to leave Time Machine alone on a separate partition and use a different partition for your "outsourced" data.