Fortiweb disk status is full, and doesn't allow me to upgrade or even upload a certificate

[Malik]

Hi, 

I have this problem where i see this message in the status: "Not available, Log will lost, please contact Fortinet"

and when i try to upgrade to another version of the firmware i get this: "Uploaded successfully, but an internal error occurred. Try checking disk space and available memory first."

and when i tried this command 

Fortiweb# diagnose hardware harddisk list name size(M) sda 15837.69 sdb 1024.46

Can anyone help with this issue. So i can manage my fortiweb?