How to Move Outlook OST Cache to a Larger Secondary Drive Safely

If your primary drive is running out of space, the Outlook OST file — which stores a cached copy of your mailbox — is often the culprit. This file can grow to tens of gigabytes, especially with large mailboxes and shared folders. Moving the OST file to a larger secondary drive frees up space on your system drive without losing any email or calendar data. This article explains the correct method to relocate the OST file, how to configure Outlook to use the new location, and what to avoid to prevent data loss.

Understanding the Outlook OST File and Why Location Matters

The Outlook OST file, or Offline Outlook Data File, is a local copy of your Exchange, Microsoft 365, or Outlook.com mailbox. Outlook uses this cached copy to provide fast access to emails, calendar items, contacts, and tasks even when you are offline. The OST file is stored by default in the hidden AppData folder at %localappdata%\Microsoft\Outlook. For users with large mailboxes or multiple shared calendars, this file can exceed 50 GB, filling the system drive quickly.

Moving the OST file to a secondary drive is safe because the file is a cache. The original data remains on the server. If you delete or move the OST file, Outlook re-downloads the entire mailbox from the server. The challenge is that Outlook does not offer a built-in setting to change the OST path after the profile is created. You must use a workaround: either create a new Outlook profile with a custom OST path or use a symbolic link to redirect the default location.

What Happens If You Move the OST File Incorrectly

Simply cutting and pasting the OST file to a new drive while Outlook is open will cause a corrupted file. Outlook will then try to repair the file, which often fails and forces a full re-download. If you copy the file instead of moving it, Outlook will create a second copy and your mailbox may appear empty or show duplicate items. The symbolic link method avoids these problems because Outlook continues to write to the same logical path, even though the physical data is on another drive.

Steps to Move the OST File Using a Symbolic Link

This method works for Outlook 2016, Outlook 2019, Outlook 2021, and Outlook for Microsoft 365 on Windows 10 and Windows 11. You need local administrator rights on the computer.

1. Find the current OST file location
   Open Outlook and go to File > Account Settings > Account Settings. On the Data Files tab, select your Exchange account and click Open File Location. This opens the folder where the OST file is stored. Note the full path and the exact file name, for example, user@domain.com.ost. Close the folder and close Outlook completely.
2. Create a folder on the secondary drive
   Open File Explorer and navigate to your secondary drive, such as D:\. Create a new folder named OutlookCache or any name you prefer. This folder will hold the moved OST file.
3. Move the OST file to the new folder
   Press Windows key + R, type %localappdata%\Microsoft\Outlook, and press Enter. Locate the OST file you identified in step 1. Cut the file (Ctrl + X) and paste it into the folder you created on the secondary drive. If you see a permission prompt, click Continue.
4. Open Command Prompt as Administrator
   Press the Windows key, type cmd, right-click Command Prompt, and select Run as administrator. Click Yes if prompted by User Account Control.
5. Create the symbolic link
   In the Command Prompt window, type the following command and press Enter:

   mklink /J "%localappdata%\Microsoft\Outlook\user@domain.com.ost" "D:\OutlookCache\user@domain.com.ost"

   Replace user@domain.com.ost with your actual file name and D:\OutlookCache with your chosen drive and folder. The /J switch creates a directory junction, which works for files as well as folders. If the command succeeds, you see a message saying Junction created.

6. Launch Outlook and verify
   Open Outlook. It should load your mailbox normally without downloading a new OST file. To confirm the file is on the secondary drive, go to the secondary drive folder and check that the file size matches the original. You can also check that the junction exists by running dir %localappdata%\Microsoft\Outlook in Command Prompt. The OST file entry will show in the listing.

Alternative Method: Create a New Outlook Profile with a Custom OST Path

If you prefer not to use symbolic links, you can create a new Outlook profile and set the OST file location during the profile creation. This method requires you to re-add your email account and reconfigure any custom settings.

1. Open Mail in Control Panel
   Press Windows key + R, type control panel, and press Enter. Change the view to Large icons and click Mail (Microsoft Outlook). If you see Mail (32-bit) or Mail (64-bit), choose the one matching your Outlook version.
2. Create a new profile
   Click Show Profiles. Under the General tab, click Add. Enter a profile name such as WorkMail and click OK. Outlook prompts you to add an email account. Enter your email address and password, then click Next. Outlook configures the account automatically.
3. Change the OST file path during setup
   When Outlook finishes configuring, it asks if you want to set up Outlook on your phone or wait. Click Done. The profile is created with the default OST path. To change the path, you must use a registry edit or the symbolic link method. This alternative does not allow you to set a custom path during setup in modern Outlook versions. Use only the symbolic link method for a clean result.

Common Mistakes and What to Avoid

Moving the OST File While Outlook Is Running

Outlook locks the OST file while it is open. If you try to move or delete the file, you will get a File in Use error. Always close Outlook and verify that the process OUTLOOK.EXE is not running in Task Manager before proceeding.

Using a Network Drive or USB Drive for the OST File

Outlook does not support OST files stored on network drives or removable drives. If you place the OST file on a network share, Outlook will fail to open the mailbox. Use only a local internal drive or an SSD connected via SATA or NVMe.

Deleting the OST File Instead of Moving It

If you delete the OST file and then create a symbolic link, Outlook will create a new empty OST file at the junction target and start a full mailbox download. This process can take hours for large mailboxes. Always move the existing OST file to the new location before creating the link.

Typing the Symbolic Link Command Incorrectly

The mklink command is case-insensitive for paths but requires exact syntax. A missing space, wrong quotation mark, or extra slash causes a Cannot create a file when that file already exists error. Double-check the path and file name before pressing Enter. If you get an error, remove the junction using rmdir “%localappdata%\Microsoft\Outlook\user@domain.com.ost” and try again.

OST File on Primary Drive vs OST File on Secondary Drive

Item	OST on Primary Drive	OST on Secondary Drive
Description	Default location in AppData folder on C drive	Relocated to a larger drive such as D or E
Disk space usage	Uses space on system drive, can cause low disk warnings	Frees up system drive space for Windows updates and apps
Performance	Fast if using an SSD; no performance gain from moving	Same performance if target drive is also an SSD
Setup complexity	No setup needed	Requires symbolic link or new profile
Risk of data loss	Low if drive does not fail	Low if symbolic link is created correctly

You can now move your Outlook OST file to a secondary drive using the symbolic link method. This frees space on your system drive without losing any mailbox data. After moving the file, verify that Outlook opens and syncs normally. For best performance, keep the OST file on an SSD. If you later need to move the file again, delete the old junction and create a new one pointing to the updated location.