Space of Disk when Importing "On My Computer data" on Macbook

Question

Space of Disk when Importing "On My Computer data" on Macbook

Trung Nguyen 0

I just install Outlook Home & Business 2024 on Macbook to replace Outlook 365. When importing "On My Computer Data", it needs the same extra space of the size of "On My Computer Data"? As I see this process is occupying space from my dish.

For example, if my email "On My Computer" is 1 TB for previous Outlook 365, it will need another 1 TB for importing "On My Computer Data" when using Outlook Home & Business 2024?

0 comments

2 answers

Your answer

Answer 1

Jayden-P 18,565 Microsoft External Staff Moderator

Hi @Trung Nguyen

As you already know, “On My Computer” folder is stored inside Outlook’s local data file and is not synced to any email server.

When you import local mail .olm file, Outlook copies that data into its local storage. This means the imported mail takes up about the same amount of disk space again.

So, if you already have 1 TB of local mail and you import another 1 TB, Outlook needs enough disk space to hold both.

After the import is complete and you have verified the data is present in Outlook, you can delete the .olm file to free up disk space. (Only do this if you no longer need the .olm file as a backup.)

One more quick check, from this article, Outlook Home & Business 2024 does not support Legacy Outlook (which means no Import/Export feature), do you have any other subscription/ license that support it?

User's image

This Import/Export feature on New Outlook Mac will be rolled out starting from June 2026. Click here for more details.

User's image

I hope this information helps. Please let me know if you need anything else.

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.

Jayden-P 18,565 Reputation points Microsoft External Staff Moderator

2026-03-27T08:09:43.7+00:00

Hi @Trung Nguyen

Have you checked the provided answer?

Any update would be great.
Trung Nguyen 0 Reputation points

2026-03-28T09:57:45.47+00:00

Thanks for your helpful information.

My Outlook did "Importing On My Computer data", and it failed due to there is no enough space during its importing process. However, it still occupies the rest of space of my dish with total 1.7 TB now. The space of my previous Outlook 365 is about 1 TB.

Now, I don't have any subcription/license support for Legacy as MS 365 is expired & I just bough Office Home & Business 2024 for using instead. I just found Office Home & Business 2024 don't have Legacy Outlook & this outlook don't support Import/Export feature too.

In this case, I plan to return back to Legacy Outlook as I was stuck in working with email outlook these days. Could you pls advise which folder/file that I can delete it safely after the importing "On My Computer data" failure when transfering from Legacy Outlook to New Outlook? Pls help indicate folder/file location on Macbook for easy follow-up.

By the way, with your provided information, Legacy Outlook will be stopped for supporting soon, and New Outlook will be used in the future. Legacy Outlook & New Outlook are different systems, and importing "On My Computer" data takes much space like that, how it will be solved if there is no enough free space for this process like my case? Or Microsoft will give any other solution then for this process without using much space of disk?

Moreover, could you pls advise how to free up space of outlook email on Macbook? It takes up much space of my disk when I use many emails in working (about 2000 new emails per day)? There is any feature reducing outlook email size on Macbook like compressing .pst file in Outlook for Window?

Thanks so much!
Jayden-P 18,565 Reputation points Microsoft External Staff Moderator

2026-03-28T15:29:46.9266667+00:00

Hi @Trung Nguyen

It is a little confusing. Previously when you used M365 subscription, did you use Legacy or New Outlook?

In your New Outlook currently (2024), do you see On my computer folders? If not, you can go to settings > General > Uncheck Hide On my computer folders.

Because from my understanding, this folder is only stored offline, you don't need to re-import it again.
Trung Nguyen 0 Reputation points

2026-03-30T07:57:46.0333333+00:00

Sorry to make you make you confused.

Previously, I used Outlook 365 for Legacy Outlook.

Currently, I use Outlook Home & Business 2024, and this version just has New Outlook (don't have Legacy Outlook. There is On My Computer folder on Outlook 2024, but it did importing "On My Computer data" automatically when I installed Outlook 2024 but failed due to the above-mentioned reason.
Jayden-P 18,565 Reputation points Microsoft External Staff Moderator

2026-03-30T10:04:56.53+00:00

Hi @Trung Nguyen

I understand your situation now, I have tested and my observation is On my computer from New Outlook and Legacy are stored separately.

So, just like you said, if the Legacy has 1TB, the New will need another 1TB because it will only copy (not move).

Only when you finish the migration, the option to remove previous copy will appear.

So right now, in your New Outlook (2024), does the On my computer folder empty or it has some emails?
Trung Nguyen 0 Reputation points

2026-03-30T13:55:18.91+00:00

Hello @Jayden-P

Thanks for your testing, I can have more understanding abut it through your test.

However, as I said, the importing “On My Computer data” failed as my Macbook doesn’t have enough space for it. And now, “On My Computer” folder is empty due to its failure.

So now, I plan to using Outlook 365 back. If I install Outlook 365 back, it can be used normally?

Moreover, due to this failure in transfering from Outlook 365 to Outlook 2024, my disk is now 1.7 TB (previously about 1TB before installing Outlook 2024). So I still have some pending questions as below (bold letter) as mentioned:

Could you pls advise which folder/file (about 0.7 TB) that I can delete it safely after the importing "On My Computer data" failure when transfering from Legacy Outlook to New Outlook? Pls help indicate folder/file location on Macbook for easy follow-up.

By the way, with your provided information, Legacy Outlook will be stopped for supporting soon, and New Outlook will be used in the future. Legacy Outlook & New Outlook are different systems, and importing "On My Computer" data takes much space like that, how it will be solved if there is no enough free space for this process like my case? Or Microsoft will give any other solution then for this process without using much space of disk?

Moreover, could you pls advise how to free up space of outlook email on Macbook? It takes up much space of my disk when I use many emails in working (about 2000 new emails per day)? There is any feature reducing outlook email size on Macbook like compressing .pst file in Outlook for Window?
Jayden-P 18,565 Reputation points Microsoft External Staff Moderator

2026-03-30T14:19:44.02+00:00

Hi @Trung Nguyen

As for now, please do not delete anything first, you need to make sure you have a backup of your email. Deleting now can cause data corruption or even data loss.

If possible, switch back to Legacy Outlook. This allows you to access the “On My Computer” folder directly. From there, you can move emails out of “On My Computer” into another folder or mailbox, This will reduce the size of the folder.

In your Finder> Go > Go to folder > ~/Library/Group Containers/UBF8T346G9.Office/Outlook/ > Outlook 15 profiles > Main profile > Delete Caches folder.

For your additional inquiries:

I have not found any announcement or article from Microsoft about migrating from Legacy to New Outlook.

Are you using Exchange, IMAP or POP account? If you are using Exchange or IMAP, you can limit the number of items sync to local.

If you are using POP, by default, it will download all emails and store offline, you can export it to .olm file and store in external drive.
Trung Nguyen 0 Reputation points

2026-03-31T05:57:36.0333333+00:00

Hi @Jayden-P

Thanks for your advice!

I'm using Exchange account. I use server email on Outlook, then working on it & remove unnecessary emails at the same time, then store necessary mails in "On My Computer" folder (for working on new & old email later) when server mailbox ups to maximum 50 GB of my server mail box.

I just change to Legacy Outlook now with LTSC 2024 instead of Outlook 365. There is normal emails in "On My Computer" folder & it works smoothly now. However, I need to delete the occupied space due to the failure in "Importing On My Computer data" when transfering from Outlook 365 (Legacy Outlook) to Outlook Home & Buisness 2024 (New Outlook). I just found 2 suspicious folders for it as below:

1st folder:

Folder name: OnMyComputerMigrationFailedItems

Size: 479.77 GB

Location: ~Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Profile 1

2nd folder:

Folder name: OMCMigrationSourceFolder

Size: 105.2 GB

Location: ~Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Profile 1

The total size of these 2 folders is nearly the size of the above failed transfering. I wonder that if I can delete these 2 folders without affecting to my current email & data? Could you pls advise?

Moreover, I don't quite understand about your wording "you can move emails out of “On My Computer” into another folder or mailbox, This will reduce the size of the folder.".

As I use Exchange account, so there is only 2 general main mailboxes, one is the server mailbox, the other is mailbox in "On My Computer". Subfoder can be made into each mailbox. For my case, it is impossible to move emails out of "On My Computer"?

As my understanding, email storage location on Macbook is stored as default by MacOS, located at ~Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles. There is only 1 folder like this, and emails coming later in "On My Computer" folder will make this folder bigger & bigger from time to time. There is no way to move this folder to other location on computer disk or external disk, is it right? If can move, it won't link old emails into Outlook? If the folder much bigger in future and there is no enough disk space for it, is there any other way to reduce it beside deleting Caches folder as you mentioned?
Jayden-P 18,565 Reputation points Microsoft External Staff Moderator

2026-03-31T07:13:38.7633333+00:00

Hi @Trung Nguyen

When I said, "you can move emails out of “On My Computer” into another folder or mailbox, this will reduce the size of the folder", I mean to transfer a small portion of the local data to server, but if it's not doable in your case, please skip it.

And the Outlook 15 Profiles folder cannot be moved to another location, if you move it, Outlook will recreate another one.

From my understanding, you can delete OnMyComputerMigrationFailedItems folder.

For further help, I recommend you contact support within Outlook Mac. Contact support within Outlook for Mac - Microsoft Support
Trung Nguyen 0 Reputation points

2026-04-01T04:47:47.3633333+00:00

Hi @Jayden-P

Noted, thanks for your kind support.

It seems that it's hard to get direct support from Microsoft. I try to solve it. It takes much time.
Jayden-P 18,565 Reputation points Microsoft External Staff Moderator

2026-04-01T06:24:33.8533333+00:00

Hi @Trung Nguyen

I hope you will be able to solve it soon.

Answer 2

Outlook for Mac stores all locally cached data in the Outlook profile on the Mac’s disk. “On My Computer” folders are local-only data and are part of that cache.

During an import of “On My Computer” data (for example from a .pst or .olm archive), Outlook writes that content into the local Outlook profile. In practical terms, this means:

The imported data will occupy roughly its full size again inside the Outlook profile (plus some overhead for indexing and database structures).
If the existing “On My Computer” data is 1 TB and another 1 TB of data is imported, the Outlook profile must be able to grow to hold the combined data. There is no in-place “reuse” of the old 1 TB; Outlook maintains its own local cache and database.
Outlook for Mac is designed as a locally cached client, and the profile location is fixed under the user’s home folder; moving the profile to an external disk or network location is not supported.

So, when importing, plan for at least the full size of the data being imported to be available as additional free disk space, plus some extra overhead.

References:

Share via

Space of Disk when Importing "On My Computer data" on Macbook

2 answers

Your answer