Time Machine in High Sierra

I would start over.

It sounds like your external drive is partitioned and one partition is using either NTFS or some FAT version.. neither are suitable for TM backups which must be done to HFS+.

So step one.. assuming your disk has nothing on it you need.. open disk utility.. re-partition it and make sure you use GUID type. If you want multiple partitions is up to you.. but you obviously now have two partitions. Hence 2 drives are mounted.

Then format the drive Mac OS extended Journaled.

Once you do that it should work ok as a TM drive.

Problem is your drive is partitioned using MBR.. as typical windows external drive.. and although you can use it on a Mac it can cause issues.. so that is why I recommend you use GUID .. but you would need to delete all the partitions to do it.

Up to you .. in the utility you can just reformat the windows side.

That is certainly what I do.. and recommend.

MBR is fine for most external drives if you don't need to boot from it.. but Apple has used GUID partitioning for a long time now.. and keeping stuff as native as possible is always good.

It will wipe out your drive.. as you must change the whole partitioning to do it.. and I frankly hate the disk utility. It is one of those stupid improvements in looks Apple makes with features removed or made extra obscure for no rational reason.. it is a utility after all.. used 10 times at most in the whole lifetime of the computer by 10% of the user base.. I would think at least 50% have never used it in their lives. And 40% probably once.

So if you happen to have a computer running older OS.. it is dead simple. For new OS you might need to read the instructions.. completely the reverse of where Apple should take people. IMHO of course.. !! Not so humble.. when discussing stupid changes!!