You do not have to worry. If it is a device without the possibility of inserting microSD cards, the call to getExternalStorageDirectory returns the route to what is called "shared storage", even if it is not external or removable:
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Note: do not be confused by the word "external" here. This directory
can better be thought as media / shared storage. It is a filesystem that
can hold a relatively large amount of data and that is shared across
all applications (does not enforce permissions). Traditionally this is
an SD card, but it could also be implemented as built-in storage in
device that is distinct from the protected internal storage and can be
mounted as a filesystem on a computer.
Translation:
Note: Do not get confused here with the word "external". You can think of
this directory as a shared medium or storage. Is a
file system that can contain a relatively large amount
large amount of data and that is shared by all applications (not
impose permits). Traditionally it has been an SD card, but
could be implemented as internal storage different from the
internal storage protected and that could be mounted as a
file system on a computer.
To differentiate if it is of one type or another you have to use isExternalStorageRemovable :
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