A Windows Defender bug creates thousands of small files that waste gigabytes of storage space on Windows 10 hard drives. The bug started with Windows Defender antivirus engine 1.1.18100.5 and will cause the C:ProgramDataMicrosoftWindows DefenderScansHistoryStore folder to be filled up with thousands of files with names that appear to be MD5 hashes. The files range in size from 600 bytes to a little over 1KB. Windows Defender users can check their version by going to Windows Security, clicking on the Settings cog icon.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-defender-bug-fills-windows-10-boot-drive-with-thousands-of-files/