Time Machine HDD is unwritable.

Bought a portable HDD from Costco. It's Seagate One Touch 5TB Portable HDD. It works with Windows and Mac per its features. I can use Time Machine to back up my data. However, I found out once it's used as Time Machine drive, it doesn't allow me to write on this drive. I tried to remove data and reformat it as APFS and/or Mac OS (Journal). At this point, the icon of the drive is in orange, the default icon of an external drive. Go to the info of this drive, it's readable and writable to me. I can drop and remove files on this drive just like others. But when I try to connect it to Time Machine for a backup drive, that's where goes wrong. So I already have a portable drive from WD as my main backup drive. The format of WD is Mac OS (Journal). Now I'm adding Seagate as 2nd one. I open up Time Machine, click add button to add Seagate. Since I already have one drive, the system asks me to use both drives or WD drive. Of course, I click the first option for both drives. Then, a window pops out says the machine is preparing. Then, boom, it is added to Time Machine line. The icon of Seagate drive turns to Time Machine look (not orange anymore). The format seems to be formatted into APFS. Then, this drive becomes unwritable. Go to info, doesn't allow me to change the settings. Now I can't drop any files in this backup drive. No matter what format I set up at the beginning, when I use it for Time Machine drive, the format is reformatted to APFS. Please help!