The root partition can become full, requiring manual file deletion. If the root is full, the device cannot to perform maintenance tasks such as content installs (AV, APP/Threat, URL, DB) or generate tech support files. To check the status of the root partition, use the show system disk-space command. Core files consume large amounts of disk space: show system files. Delete large core files: delete core management-plane file <filename>.
Delete rotated files and files with extention .old as follows. These files contain monitoring details and service related logs on the firewall. Hence they can be deleted safely if you don't need them. If TAC investigates an ongoing issue, you may prefer to keep them until you upload the tech support file to the case manager.