Storing data on a external drive used for backup Is it safe to store data on HDD which I selected to use as a backup device in time machine? Or should I add another partition for that?

I do not know how does time machine behave when there is not enough space. Is it going to delete whatever it sees or just content from Backups.backupd folder.
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.