When attempting to reorganize your SharePoint 2013 topology you may also want to specify a new path for your search Index. The reason for this might be that you want to have a dedicated drive for a large or busy search index. You can find the general technique for modifying the topology here: http://mmman.itgroove.net/2013/03/changing-the-sharepoint-2013-search-topology/

Looking up the involved New-SPEnterpriseSearchIndexComponent at http://technet.microsoft.com/en-us/library/ff607766(v=office.14).aspx we can see the new file path is designated by the -IndexLocation switch. An example would be:

$serv = "MYSERVERNAME"
$NewIndexPath = "E:SharePointIndex"
New-SPEnterpriseSearchIndexComponent -SearchTopology $newTopology -SearchServiceInstance $serv -RootDirectory $NewIndexPath

But wait- if the destination path does not exist on the source server, it will error out with “Could not find part of the path”. At first I thought it was permissions, but having checked that the new destination folder had appropriate permissions (WSS_WPG with modify permissions), I was still scratching my head. It became clear this was a bug, as described on http://office.microsoft.com/en-ca/help/sharepoint-server-2013-known-issues-HA102919021.aspx :

“New-SPEnterpriseSearchIndexComponent checks the existence of RootDirectory in the wrong server

You want to add a new index component to the search topology, and want to specify a non-default root directory for the index files. For example:

**Keith Note: Do not use this PowerShell, it has typos quoted as-is from TechNet. Provided for reference only:

New-SPEnterpriseSearchIndexComponent -SearchTopology $t -SearchServiceInstance $ssi -IndexPartition 1 -RootDirectory ""d:index4"

The cmdlet might fail with the following error message because it incorrectly checks if the indicated root directory exists on the server the cmdlet is run on:

Cannot bind parameter ‘RootDirectory’ to the target. Exception setting “”RootDirectory””: “”Could not find a part of the path ‘d:index4’.”

Workaround You can create the new index component using the following procedure instead:”

$host02 = (Get-SPServer "<Name of server>").Name
$ssa = Get-SPEnterpriseSearchserviceApplication
$t = $ssa.ActiveTopology.Clone()
$ic = (New-Object Microsoft.Office.Server.Search.Administration.Topology.IndexComponent $host02,1);
$ic.RootDirectory ="d:index4"
$t.AddComponent($ic)

Note: In the code snippet on the MS bug report, there are three typos (fixed in the PowerShell shown above)

1. missing space between SP-GetServerName and parameter
$host02 = (Get-SPServer””).Name
should be
$host02 = (Get-SPServer “”).Name

2. double quotes in beginning of the path declaration:
$ic.RootDirectory =””d:index4″
should be:
$ic.RootDirectory =”d:index4″

3. extra quote at the end:
$t.AddComponent($ic)”
should be:
$t.AddComponent($ic)

Note 2: You must permit TCP port 808 through your Windows Firewall on the source and destination servers or the topology move in general won’t work!