Answer by Rick James for AWS MariaDB reports via CloudWatch that disk space free storage is 0 but 40% unaccounted for

Dump & load --

1. Dump -- copies data to another part of disk, thereby consuming disk space.
2. Load -- copies data from the dump to the database (making a new db), still more disk space
3. Free up the disk space from step 1.
4. DROP DATABASE may free up more space -- depends on innodb_file_per_table.

However, to take "hours" to do step 3 makes me think that the OS (or the complex disk system that AWS has) is slow to release the space.

Bottom line: Live with it.