Its simply the way Acer chose to setup the PC, & name the drives.
You can locate/move your data files to the D drive, or use D for any
backups, or when installing a program choose Custom install & install to D
rather than C

Hi David,

D: is simply storage space. Think of it as having two bookshelves instead of
one to store books on. Create folders as desired on it to store files of
your choosing. If desired, you can also remap things like your User
Documents files to automatically go there (just right click the folder,
choose properties and check the location tab).