Before you repair a PST file: eight steps that protect your mail

Before you repair a PST, close Outlook completely and make a copy of the file somewhere else. Then check where the file lives and whether the disk is healthy, look for older copies, confirm whether it is a PST or an OST, and read the error carefully. Only then run a repair, and run it on a copy. The rest of this article explains each step and why it matters.

The reason for the caution is in Microsoft’s own description of the Inbox Repair Tool (SCANPST). It mostly fixes the file’s internal structures, and when it finds an unreadable block that belongs to a message, “the item will be removed when it is validated” (Microsoft Learn). That is a reasonable design for a repair tool. It also means a repair can leave you with less than you started with, so you want an untouched copy before it runs.

1. Close Outlook, and make sure it is really closed

Outlook keeps a PST open while it runs, and other programs can keep Outlook running after its window closes: add-ins, phone sync software and search indexing are common ones. Copying a file in that state can fail with a “locked” error or produce an incomplete copy.

Close Outlook, wait a minute, then open Task Manager (Ctrl+Shift+Esc) and check that no Outlook or OUTLOOK.EXE process is left. If one is, end it, or restart the computer and do not open Outlook. Microsoft’s repair article starts the same way: “Exit Outlook” before running the tool (Microsoft Support).

2. Copy the file somewhere else

Copy the PST to a different drive: an external disk, another internal drive or a network location. Check that the copy has the same size as the original. A PST is one large file, and a copy that stops halfway leaves you with a second damaged file rather than a backup.

This is the most important step in the whole article. SCANPST offers to make a .bak backup, and you should leave that option ticked, but it is not a substitute for your own copy. The .bak sits next to the original on the same disk, so it shares whatever happened to that disk, and it only exists if the option was left on. A copy you made yourself, on another drive, before anything ran, is the only one you can be sure of.

If the PST is large and you are short of space, make the copy anyway. It is the step that makes every other one reversible.

3. Check where the file lives

Where the PST is stored can be the cause of the damage, and repairing it in the same place can repeat it.

• On a network drive. Microsoft states that using a PST over a network “may result in a corrupted .pst file if the connection degrades or fails”, and supports it only in one narrow remote desktop setup (Microsoft Learn). SCANPST’s multi-pass mode has “no support for networked .pst files” (Microsoft Learn). Copy the file to a local disk first and work there.
• In a OneDrive, Dropbox or other synced folder. OneDrive “cannot sync” a file while it is open, Outlook data files included (Microsoft Support), so the file is constantly being written by one program while another tries to copy it. Pause syncing, close Outlook, let the sync finish, and copy the file out of the synced folder.
• On a USB drive. If the drive was unplugged while Outlook had the file open, that is a likely cause. Work on a copy on an internal disk.

4. Check the disk before you trust it

If the disk is failing, every read and write makes things worse, and a repair writes a lot.

• On Windows 11, Settings › System › Storage › Advanced storage settings › Disks & volumes › Properties shows drive health for supported drives. Microsoft notes this warning covers NVMe SSDs only, not SATA SSDs or hard drives (Microsoft Support).
• In PowerShell, Get-PhysicalDisk reports a HealthStatus of Healthy, Warning, Unhealthy or Unknown.
• chkdsk with no options only reports: “If used without parameters, chkdsk displays only the status of the volume and doesn’t fix any errors” (Microsoft Learn). With /f or /r it changes the disk, so copy your data off first.

If the disk reports problems, copy the PST (and anything else important) to a healthy disk before doing anything else, and do the repair there. For a disk that is clicking, disappearing or very slow, stop using it and consider professional data recovery.

5. Look for an older copy

An undamaged older copy may get you further than any repair. Look for:

• Previous Versions or File History. Right-click the folder that holds the PST and choose Restore previous versions. Use Restore to… to save the old version somewhere else; Microsoft warns that a plain restore replaces the current file and “the replacement can’t be undone” (Microsoft Support).
• Backups: an external drive, a backup service, an old computer, or a copy someone made before a migration.
• The mailbox itself. If the mail came from a Microsoft 365, Exchange, Gmail or IMAP account, much of it may still be on the server.

Even an old copy is useful: you can compare it with the repaired file and fill gaps from it.

6. Find out whether it is a PST or an OST

The file name ending matters:

• A .pst is the only copy of what is in it, unless you made others.
• An .ost is a local copy of a mailbox that lives on a server. Microsoft’s advice for a damaged OST is to delete it, after which “Outlook will recreate the offline Outlook Data File (.ost) the next time you open Outlook” (Microsoft Support), downloading everything again.

The exception is an OST whose mailbox no longer exists: a closed account, a former employer’s mailbox, or a computer that was offline for a long time with mail that never reached the server. Then the OST is the only copy, and it should be treated like a PST: copy it first and do not let Outlook rebuild over it. OST to PST turns it into a PST that opens anywhere.

7. Read the error before choosing a fix

Not every error means a damaged file.

• “Outlook data file cannot be accessed” is often a file that was moved, renamed or is on a drive that is not connected, or a profile problem. Our guide to that error goes through the simple causes first.
• “The file is not an Outlook data file” or “Errors have been detected in the file” usually do mean structural damage.
• “has reached its maximum size” means the file is full, not broken. Old ANSI-format PSTs stop at about 2 GB, and current ones at 50 GB by default (Microsoft Learn). See PST file too large.
• Mail that seems to be missing is more often hidden, filtered or archived than lost. Twelve causes to check.

8. Repair a copy, and keep the original

With a copy safe elsewhere, you can repair:

1. SCANPST is free and already installed with classic Outlook. In Microsoft 365 on 64-bit Windows it is usually at C:\Program Files\Microsoft Office\root\Office16\SCANPST.EXE; our SCANPST guide lists other locations. Leave the backup option ticked. Microsoft says you “may need to run the tool several times.” Since Outlook 2016 version 1807, running it from a command prompt with -rescan repeats the passes automatically until the results stop changing, and -no repair scans without changing anything (Microsoft Learn).
2. Afterwards, open the file in Outlook, press Ctrl+6 for the folder list, and look in Lost and Found for items SCANPST could not place. Microsoft warns that “items that are missing from the Lost and Found folder may be beyond repair.”
3. If SCANPST gives up, loops, or Outlook still cannot open the result, a tool that copies readable messages into a new file can recover more, and your untouched copy means you can still try one.

Mailtria’s PST repair works that way. Its free health check reads the whole file in your browser, without uploading it, and reports what is damaged and how many messages can be saved before anything is written. The repair then copies every readable message into a new PST, checks each one against the damaged file, and lists any it could not save. It never edits the original. It does not support password-protected or old ANSI-format PST files, and no tool can bring back data that has been overwritten.

After the repair

Do not delete the damaged original or your copy straight away. First check the result: open it, compare folder counts with what you expected, and look for the messages that mattered most. If anything is missing, the original is still there for a second attempt with another tool.

Sources

• Microsoft Support: Repair Outlook Data Files (.pst and .ost), OneDrive: this file can’t be synced, Drive health warnings, Backup and restore with File History.
• Microsoft Learn: How to repair a personal folder file, Inbox Repair tool runs multiple passes, Limits to using PST files over LAN and WAN links, Configure size limit for PST and OST files, chkdsk, Get-PhysicalDisk.