An FTP server we manage had low disk space and it looked like 60GB were missing from this server. Upon investigation we found the following folder (which was hidden) had files taking up the disk space:
C:\ProgramData\Microsoft\Windows\WER\ReportQueue
This folder is used by windows error reporting, the replacement for Dr. Watson. It was keeping reports for all the service failures this machine had experienced, so around 60GB of space was consumed by these reports.
You can safely delete these reports, however you should look into what was failing that caused them to be created in the first place. in our case it was a service that had failed to start. These reports had diagnostic info that you could have used to solve the problem, or send to Microsoft for analysis.
You can disable the error reporting by opening the control panel in classic mode, opening the problem reports and solutions, click advanced and change problem reporting to Off.