Disk partition not added

Possible reasons why a disk partition is not added to Site24x7 for monitoring

Only the first 25 disk partitions will be added for monitoring.
For Linux, only the local disk partitions present while executing the following command will be added:
df -l -T

The 'df' utility will display disk details only for disks that the user executing the command has permission to access. Therefore, if the agent is installed in 'install as root, run as non-root' mode or in non-root mode, it may not have access to the particular disk.
Read this article to know how disk utilization is calculated for a Linux monitor.
For Windows, the following WMI query is used to discover the disk partitions:
Select Name, Size, FreeSpace from Win32_LogicalDisk where DriveType=3
If 'No Data' is shown in the Site24x7 web client, refer this article to troubleshoot.