did you copy this data to the iXpand from a Mac? Of so you need to empty the trash before disconnecting the drive before it will free the space.
If this was done on a windows computer there may be other reasons however formatting the iXpand should resolve the issue.
Backup and data you do nto want to delete and see the knowledgebase article below for instructions on how to format. Use exFAT as the file system when you format. And if you are using a mac make sure and use Master Boot Record for the partition type.
Formatting on Mac
https://kb.sandisk.com/app/answers/detail/a_id/203/
Formatting on PC
