mds_stores prevents reformating of external hard drive

My external hard drive I use for back-up is formated in HFS, hence I cannot use the drive to make new back-ups. I now tried to format it to APFS with Disk Utility but it fails everytime because the external drive is used by the mds_stores process.

I've tried to disable my spotlight, reduced the spotlight index data and force quit mds_stores, but it keeps launching the second after I force quit it once by external drive is connected. Does anyone have a solution for this?