Ok, here is the solution to format only C: drive during OSD and keep other drives data unaffected.
1. Create a default task sequence for OS deployment first. Edit it after creation and remove the "Format and Partition Disk" step. You can't use this step without wiping the disk at all.
2. In the "Apply Operating System Image" step, make the following modifications before deployment: Set the destination to a specific Disk 0 and Partition 1 (for example).
3. With these changes, the partition configured will be used. The data on other partitions or disks will be saved this way! During the "Apply Operating System Image" step, the configured Disk and Partition will be formatted first before installation.