Exclude the desired disk partition via Configuration Rules: Exclude specific disk partitions from monitoring using rules. You can mention the disk partition name as such, say C: to exclude it from monitoring. Or, you can use RegEx pattern(s), say /snap.*  to exclude all disk partitions starting with /snap. 

This option can be done across servers.

Use Skip Alert for the individual disk partitions: Under the Disks tab (Server > Server Monitor > Servers > click on the server monitor > Disks), click on the pencil icon under Action for the disk partition that needs to be excluded. Ensure the option Skip Alert is set to Yes.

Disable/remove threshold configuration for disk partitions globally in server threshold profile: Go to the respective server monitor's threshold profile (Server > Server Monitor > Servers > click on the server monitor > hover on the hamburger icon beside the display name > Edit) and ensure the option Notify when Disk Partition is Down is set to No.