Why is iTunes putting music on my C drive on Windows 10 when I have it set to save it on another internal drive?

Traditionally the A: and B: drive letter designations are for floppy discs. iTunes may have an aversion to using them even if they are connected to more reasonably sized storage volumes. Recent versions of iTunes may also have trouble retaining the path to the media folder when it isn't a subfolder of the iTunes folder that holds your iTunes Library.itl database file. Changing the media folder location changes where new content is stored, but doesn't move anything from a previous location. If you consolidate to the new path the original copies still take up space on the old path. See Make a split library portable - Apple Community for more.

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Your library appears to be split already, judging by the media folder location. It should be possible to manipulate the current layout of your library into the portable shape rather than starting over. That is the aim of the user tip. Is the A: drive a standalone physical volume, or some form of mapping? If you look at a typical track in your library, or better perhaps your most recent import, you can use the Song Info (Ctrl+i) > File tab to see where it is stored. Perhaps the content on C: is no longer part of the active library? Before doing anything it is probably best to grab another external drive large enough that you can backup all your content to it.

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It is generally a good idea to let iTunes make copies in the media folder of content you add from removable drives. Placing content somewhere inside the media folder before you add it to iTunes avoids any unwanted duplication on local storage. Using the default layout of an iTunes folder that contains an iTunes Media folder, which in turn holds all media connected to the library, makes it easier to move the library between drives and computers when the need arises. How easy it is to make that transition depends in part on how large the library is and how much free space you have on the drive where you want the library to live. One method I have involves making new copies of everything temporarily, while another uses a script to move the files and reconnect the library to the new path so is potentially more space efficient but requires using an additional tool.

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