In SGOS 6.x, you can view this information in the Management Console. Select Statistics > System > Resources > Disk Use.
The lower right of the screen displays information for maximum objects supported and cached objects on the system. These represent accumulative statistics across all disks.
In SGOS 5.x, you can view this information on advanced URL page. Browse to https://<IP_address>:8082/CE/Statistics.
The object count is the number of cached objects on the box. This number represents all objects on all disks in the ProxySG appliance.
To view the number of maximum cacheable number of objects supported per disk, browse to https://<IP_address>:8082/CE/Statistics/Disk1.
To view stats for Disk2, change "Disk1" to Disk2.