partitioning HardDrive question

hey guys, i am looking to reformat my 09 MBP, but i want to back up my data with time machine before i do so (which i've never used before).
my question is - if i want to use an external harddrive with data already on it, can i create a partition for time machine to use without losing that data on the EHD? or would partitioning format and delete everything on the external ? thanks for any help

You don't have to create a partition on the external hard drive. As long as the external is formatted to HFS Extended Journaled, Time Machine will write the backup to the drive without overwriting any of the other files. Of course this assumes the external hard drive has enough space for the TM backup to be written. In any event, TM will tell you if there is not enough room.