Task: Move search index from D: drive to E: drive on the existing Index Servers.
Search topology:

Preparation:

#Discover current search topology information with documentation.

$ssa = Get-SPEnterpriseSearchServiceApplication
$old = Get-SPEnterpriseSearchTopology -SearchApplication $ssa -Active
Get-SPEnterpriseSearchServiceApplication >C:\SSA_Info.txt
Get-SPEnterpriseSearchComponent -SearchTopology $old >c:\oldtopology.txt
Get-SPEnterpriseSearchStatus -SearchApplication $ssa

#Define parameters for the move.

<# $hostA = Get-SPEnterpriseSearchServiceInstance -Identity "Server1" $hostB = Get-SPEnterpriseSearchServiceInstance -Identity "Server2" #>
$hostC = Get-SPEnterpriseSearchServiceInstance -Identity "Server3"
$hostD = Get-SPEnterpriseSearchServiceInstance -Identity "Server4"
$hostE = Get-SPEnterpriseSearchServiceInstance -Identity "Server5"
$hostF = Get-SPEnterpriseSearchServiceInstance -Identity "Server6"
$indexPath = "E:\Search_Index01"

#Clone the current active topology

$clone = New-SPEnterpriseSearchTopology -SearchApplication $ssa -Clone -SearchTopology $old

#To the cloned topology, add new Index components using new path.

New-SPEnterpriseSearchIndexComponent -SearchTopology $clone -SearchServiceInstance $hostC -IndexPartition 0 -RootDirectory $indexPath
New-SPEnterpriseSearchIndexComponent -SearchTopology $clone -SearchServiceInstance $hostD -IndexPartition 0 -RootDirectory $indexPath
New-SPEnterpriseSearchIndexComponent -SearchTopology $clone -SearchServiceInstance $hostE -IndexPartition 1 -RootDirectory $indexPath
New-SPEnterpriseSearchIndexComponent -SearchTopology $clone -SearchServiceInstance $hostF -IndexPartition 1 -RootDirectory $indexPath

#Set the new (cloned) topology as active. This step takes the most time.

Set-SPEnterpriseSearchTopology -Identity $clone

#Discover the component info in the new topology.

Get-SPEnterpriseSearchComponent -SearchTopology $clone >c:\InterimTopology.txt

#Confirm that all the components are ready. Also check in the Central Admin Site for the status.

Get-SPEnterpriseSearchStatus -SearchApplication $ssa

Remove the older index components. ONLY DO THIS AFTER NEWLY ADDED INDEX COMPONENTS ARE ACTIVE. You need to clone the latest topology and modify the new cloned topology. Then set the new clone as active when modification is done. Double Check in the InterimTopology.txt the info on the components to identify the components to remove.

IndexPartitionOrdinal : 0
RootDirectory : E:\Search_Index01
ComponentId :
TopologyId :
ServerId :
Name : IndexComponent4
ServerName : SearchServer04

$active = Get-SPEnterpriseSearchTopology -SearchApplication $ssa -Active
$newclone = New-SPEnterpriseSearchTopology -SearchApplication $ssa -Clone -SearchTopology $active

$comp1=Get-SPEnterpriseSearchComponent -SearchTopology $newclone | ? {$_.Name -eq "IndexComponent1"}
$comp2=Get-SPEnterpriseSearchComponent -SearchTopology $newclone | ? {$_.Name -eq "IndexComponent2"}
$comp3=Get-SPEnterpriseSearchComponent -SearchTopology $newclone | ? {$_.Name -eq "IndexComponent3"}
$comp4=Get-SPEnterpriseSearchComponent -SearchTopology $newclone | ? {$_.Name -eq "IndexComponent4"}

Remove-SPEnterpriseSearchComponent -Identity $comp1 -SearchTopology $newclone
Remove-SPEnterpriseSearchComponent -Identity $comp2 -SearchTopology $newclone
Remove-SPEnterpriseSearchComponent -Identity $comp3 -SearchTopology $newclone
Remove-SPEnterpriseSearchComponent -Identity $comp4 -SearchTopology $newclone

Set-SPEnterpriseSearchTopology -Identity $newclone

# Optionally, you can use a for loop to run through a list of components to remove. But I prefer the lines above as it shows explicitly the actual action to take.

#Confirm that all the components are ready. Also check in the Central Admin Site for the status.

Get-SPEnterpriseSearchStatus -SearchApplication $ssa

#Discover the component info in the final topology.

Get-SPEnterpriseSearchComponent -SearchTopology $newclone >c:\FinalTopology.txt

#Remove the old topologies

Get-SPEnterpriseSearchTopology -SearchApplication $ssa
Remove-SPEnterpriseSearchTopology -Identity < >