Outlook Search Index Location: How to Move OST Index Out of System Drive

When your system drive fills up, the Outlook search index and OST files are often the largest hidden culprits. By default, Windows stores the search index database and the offline Outlook data file (OST) on the same drive as the operating system. This article explains how to relocate both the OST file and the Windows Search index to another drive, freeing space on your C drive. You will learn the exact steps to move the index database, change the OST location, and verify that Outlook search still works correctly after the move.

Why the Search Index and OST File Fill the System Drive

Outlook uses two separate storage components that both default to the system drive. The Windows Search index contains a database of all indexed Outlook items, including email messages, calendar entries, and contacts. This database grows over time, often reaching several gigabytes. The OST file is a cached copy of your Exchange or Microsoft 365 mailbox stored locally. It can exceed 50 GB for large mailboxes. Both files reside in user profile folders on the C drive by default, consuming disk space that may be needed for other applications.

Moving these files to another drive does not affect Outlook functionality. Windows Search will continue to index new items, and Outlook will sync with the server using the relocated OST file. The only requirement is that the target drive must be a local fixed drive, not a network share or removable USB drive. The target drive should also have enough free space to accommodate the current size of the index and OST file plus future growth.

Default Storage Paths

The Windows Search index database is stored at %ProgramData%\Microsoft\Search\Data\Applications\Windows on the system drive. The OST file for each Outlook profile is stored at %LOCALAPPDATA%\Microsoft\Outlook. Both paths are hidden by default. You can view them by typing the paths into File Explorer or by using environment variables in the Run dialog. Knowing these default locations helps you confirm that the move was successful.

Steps to Move the Windows Search Index to Another Drive

1. Open Indexing Options
   Press the Windows key, type Indexing Options, and select the result. Click Advanced at the bottom of the Indexing Options window. If prompted by User Account Control, click Yes.
2. Stop indexing temporarily
   In the Advanced Options dialog, go to the Index Settings tab. Click the Rebuild button. This stops indexing and clears the current index. Do not close the dialog yet.
3. Change the index location
   Switch to the Index Location tab. In the New location field, type or browse to a folder on the target drive, for example D:\OutlookIndex. The folder must exist before you set it. Click OK. Windows Search will move the index database to the new location and restart indexing automatically.
4. Rebuild the index
   After the location change, Windows Search begins a full rebuild of the index. This can take several hours depending on the number of items. You can continue using Outlook during the rebuild, but search results will be incomplete until the rebuild finishes. To monitor progress, open Indexing Options and look at the Indexing complete status.

Steps to Move the OST File to Another Drive

1. Close Outlook
   Make sure Outlook is completely closed. Check Task Manager to confirm that no Outlook process is running.
2. Open Account Settings
   Open Outlook. Go to File > Account Settings > Account Settings. Select your Exchange or Microsoft 365 account and click Change.
3. Access the OST file location setting
   Click More Settings. Go to the Advanced tab. Under Outlook Data File Settings, click Settings. You will see the current path of the OST file. Note this path so you can copy the existing file later.
4. Change the OST file path
   Click Browse and navigate to a folder on the target drive, such as D:\OutlookData. Type a name for the new OST file or accept the default. Click Open, then click OK in each dialog to save the change. Outlook will show a message that it needs to restart to create the new OST file.
5. Restart Outlook and sync
   Close and reopen Outlook. Outlook will create a new empty OST file at the new location and begin downloading a fresh copy of your mailbox from the server. This process can take a while depending on mailbox size and network speed. The old OST file remains on the C drive and can be deleted after you confirm that the new OST file works correctly.

If the Move Fails or Search Stops Working

Index location change is grayed out

If the New location field in Indexing Options is disabled, the Windows Search service may be running with restricted permissions. Restart the Windows Search service: open Services.msc, right-click Windows Search, select Restart. Then try the location change again.

Outlook search returns no results after moving the index

This usually means the index rebuild has not finished. Open Indexing Options and check the status. If the index is stuck or shows errors, click Advanced and then Rebuild again. Also verify that the new index folder is not set to read-only.

OST file change does not take effect

If the OST file is still at the old location after changing the path, you may have multiple Outlook profiles. Repeat the steps for the correct profile. Also ensure that the target folder has full read/write permissions for your user account. Right-click the folder, select Properties, go to the Security tab, and confirm that your account has Modify permission.

Old OST file cannot be deleted

If the old OST file is locked, restart the computer and delete it immediately after booting before opening Outlook. Alternatively, use a tool like LockHunter to unlock the file. Do not delete the file while Outlook is running.

Cached Exchange Mode vs Online Mode: Impact on Index Location

Item	Cached Exchange Mode	Online Mode
OST file used	Yes, a local copy is stored and synced	No OST file; data is fetched from server
Search index location	Windows Search index on local drive	Server-side search; no local index
Disk space usage	High – OST file + search index	Low – no local data except temporary cache
Search speed	Fast – local index	Slower – depends on server and network
Offline access	Full offline access to synced items	No offline access

If you rarely work offline and have a fast connection, switching to Online Mode eliminates the OST file entirely. Go to File > Account Settings > Account Settings, select your account, click Change, then slide the Cached Exchange Mode toggle to Off. This frees up the OST file space immediately, but search will use the server index instead of the local Windows Search index.

After moving the index and OST file to another drive, you can safely recover space on the C drive by deleting the old files. The new locations will persist across Outlook updates and Windows updates. If you ever need to move the files again, repeat the same steps. This process gives you control over disk usage without losing any data or search functionality.