Ensure that disk is fully functional (no physically damages) and is accessible by Operating System.
Ensure you are not erasing the system disk or the disk KillDisk launched from (application won't let you erase these
disks).
Data still found after a Wipe operation
The Wipe operation sanitizes the only data that has already been deleted and not visible by Operating System. To sanitize ALL
the data including existing files and the Operating System itself, use the Erase Disk operation.
Erased the wrong disk
Stop erase operation as soon as possible. Once the data completely sanitized, it won't longer be accessible. Use a tool like
Active@ File Recovery (https://www.file-recovery.com) to
recover remains of data that has not been sanitized yet.