Out of the box, the drive is formatted with exFAT and an allocation size of 1 MB. That means that any file less than 1 MB takes up 1 MB of space. 1 MB may not seem like much on a 250 GB-2 TB drive, but with the myriad of small files that show up on backups taking up 1,000x more space than normal, it adds up.
https://www.amazon.com/SanDisk-Extreme-Portable-External-SDSSDE60-2T00-G25/dp/B078T9SZ3K