Help with making the query work

Hi Arslan11 

You may get a faster response if you don't ask me directly (I'm often busy with my job, and look here infrequently at times) also others may have alternative ideas and methods to share.

  
Anyway, you can probably achieve the outcome with this, I've added a capacity case statement, that allows for a specific computer and drive letter size, plus a default option for the unspecified computers :

Perf
// set up filters
| where ObjectName == "LogicalDisk" and CounterName == "% Free Space" 
| where strlen(InstanceName) == 2 and InstanceName contains ":" 
// exclude ALL these named computers 
| where Computer !in ("net-fs3.networkhg.org.uk","NET-FS1.networkhg.org.uk","NET-SQL3.networkhg.org.uk",
                    "NET-EDMLIVEDB1.networkhg.org.uk","NET-EDM_KOFAX1.networkhg.org.uk")
// Show all
| summarize Free_Space = min(CounterValue) by Computer , InstanceName
// Exclude these drive / Computer combinations
// Use a "1" to denaote an exclude else "0"
| extend ComputerList = case(
                            Computer == "NET-REPAIR2.networkhg.org.uk" and InstanceName == "E:",1,
                            Computer == "NH-E2016-01.networkhg.org.uk" and InstanceName == "E:",1,
                            Computer == "NH-E2016-02.networkhg.org.uk" and InstanceName == "E:",1,   
                            Computer == "net-boxi1.networkhg.org.uk"   and InstanceName == "D:",1,
                         // Computer == "NET-SQL2"                     and InstanceName == "G:",1,
                            Computer == "RETAILVM01"                   and InstanceName == "D:",1,
                            //else zero
                            0)
| where Computer startswith "R" or Computer  startswith "SQL" 
| where ComputerList !=1
// Set a required capacity for a specifc computer and drive letter or use a default value like 10% 
| extend driveCapacityFilter = case(
                            Computer == "NET-SQL2"   and InstanceName == "G:", 6,   
                         // Computer == "RETAILVM01" and InstanceName == "G:", 80,                   
                            // else use default value i.e.10%
                            10)
| where Free_Space < driveCapacityFilter
| project-away ComputerList