How to securely erase a bad fusion drive with hidden partition? Time Machine can access data, not visible otherwise.

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How to securely erase a bad fusion drive with hidden partition? Time Machine can access data, not visible otherwise.

iMac OS 10.15.3, Built 2013.

was previously Filevaulted

This iMac's fusion drive split and I have since repaired it. The drive appears bad. I erased the drive - at that time the operating system was Mojave, but added Catalina after erase. But on a recent Time Machine backup all the data shows as STILL ON the computer, and not visibly accessible except on a Time Machine back up I recently made. It is just like I never erased it.

BRIEF BACKSTORY: On another iMac I had a little issue with the Time Machine so I thought I would test the issue out by backing up the 2013 iMac (which was erased and NO NEW DATA was added) via Time Machine and just see what the result was. The result really surprised me:

Time Machine showed:

Macintosh HD
Macintosh HD - Data
Macintosh HD-1 ---- and this showed ALL my previous documents which should have been wiped when I erased the machine - completely accessible.
Recovery

QUESTION: How can I erase my data on this computer? Since the data shows on the Time Machine I'm thinking there might be some way to delete this data .... maybe via Thunderbolt cable and another Mac? Or? Any and all suggestions welcome. Happy to answer any further questions if needed.

Other things I've tried:

I have again erased the data. (Disk Recovery Mode / Disk Utilities /erased Macintosh HD-Data / followed by Macintosh HD (unmounted or mounted} Apple Sr. Advisor said it didn't matter) - still have the same issue.

iMac Line (2012 and Later)

Posted on Feb 8, 2020 12:38 PM

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Answer 1

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HWTech

Level 9

56,486 points

Feb 9, 2020 12:23 PM in response to ckralc

Both the SSD and Hard Drive appear to be Ok.

Either you are not erasing the base of the Fusion Drive which will remove all the partitions or volumes from the drive or you need to erase your TM drive before starting a new backup as suggested by @Kappy. Or it could be both of these things.

A TM drive should contain just a single GUID partition as MacOS Extended (Journaled). A TM drive should only contain a backup from a single computer and it should not contain any other data on it to minimize the possibility of problems. Please follow @Kappy's advice since he is much more knowledgeable about TM than I am since I've never personally used TM before.

If the base of the Fusion Drive is erased and you have properly erased the drive to be used for TM, then you should not have any problems as you've described.

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Answer 2

HWTech

Level 9

56,486 points

Feb 8, 2020 1:00 PM in response to ckralc

To destroy all the existing partitions and volumes on the drive you need to select the actual physical drive when you erase it with Disk Utility while booted into Recovery Mode, Internet Recovery Mode (Command + Option + R), or from a bootable macOS USB installer. With more recent versions of macOS you need to click on "View" within Disk Utility and select "Show All Devices" before the physical drive appears on the left pane of Disk Utility. If you have a Fusion Drive setup, then you want to actually select the actual base of the Fusion Drive which contains all the other partitions/volumes. IIRC the base of the Fusion Drive will actually say "Fusion Drive".

If you are installing macOS 10.14+, then you want to erase the drive as GUID partition and APFS (top option), otherwise you want to erase it as GUID partition and MacOS Extended (Journaled).

Before you do this you may want to check the health of your hard drive in case it may be failing and might be what caused the issue with the Fusion Drive in the first place. Run DriveDX and post the health reports for both the SSD and the Hard Drive here using the "Additional Text" icon which looks like a piece of paper.

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Answer 3

ckralc Author

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5 points

Feb 8, 2020 2:05 PM in response to Kappy

My goal is to securely erase my iMac which has a Fusion Drive. A couple of months ago I erased the Fusion Drive via Recovery Mode / Disk Utilities and added Catalina. I did not add any data. None of the data I restored shows in Documents in Finder. So I thought my data was erased and securely.

I wanted to test what a Time Machine backup looked like on Catalina so since this iMac was available, I hooked an external drive up to it and added it on as a Time Machine. I did not expect to see my data. After it backed up, the Time Machine backup had the following four folders:

Macintosh HD
Macintosh HD - Data
Macintosh HD-1 ---- and this showed ALL my previous documents which should have been wiped when I erased the machine - completely accessible.
Recovery

In Recovery Mode I cannot see Macintosh HD-1 and erasing the volumes Macintosh HD and Macintosh HD-Data does not erase the data I can access via Macintosh HD -1 (to me this seems like a hidden partition because I have not been able to erase it). I cannot erase the Fusion Drive in Recovery Mode as it will not mount.

I help this clarifies things.

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Answer 4

ckralc Author

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5 points

Feb 10, 2020 7:32 AM in response to HWTech

HWTech,

Based on your and Kappy's thoughts and suggestions I erased my Time Machine and reformatted it. Then I erased the iMac in Disk Utilities as follows:

Tried to erase the physical drive - but it is still unmounted
erased Macintosh HD - Data - Data. (Assuming this to be Macintosh HD - Data)
erased Macintosh HD - Data
erased the container

Then I Installed Catalina and when I went back into Disk Utilities the iMac now shows the volumes as:

Macintosh HD
Macintosh HD - Data
Still no access to mount and erase the physical disk

Then I erased the external drive I was using for the Time Machine and then used it to back up the iMac. The results of this unencrypted backup - a NORMAL Backup with NONE of my DATA showing. So from now on when setting up a Time Machine for a new computer - it will be fresh and/or erased of any other computer's Time Machine Data. Many thanks for your help.

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Answer 5

ckralc Author

Level 1

5 points

Feb 9, 2020 5:05 PM in response to Kappy

Kappy,

Yes, the Time Machine I used did have other backups from other machines on it.

The backup drive the Time Machine is on was formatted: Mac OS Extended ( Journaled, Encrypted).

Macintosh HD-1 is NOT a volume on the Fusion Drive that I can see (this is what made me think it was hidden). I do have a 1TB Fusion Drive - I'm guessing I have a 24 GB SSD - I don't know exactly where to look for the size. This iMac was built in 2013.

My Fusion Drive IS formatted APFS. Should I erase - if I can .... so far I haven't been able to mount the physical drive to erase it, but did manage to erase the Macintosh HD and Macintosh HD - DATA Volumes ..... however my FUSION DRIVE in Disk Utilities now looks very peculiar ( I have Macintosh HD - DATA and Macintosh HD Data-Data as volumes and no Macintosh HD) ... see images:

and

The Macintosh HD-1 -(yesterday) when I first looked at it contained among other things the user data I thought had been erased from the iMac. HOWEVER today when I view the backup there is only Macintosh HD, Macintosh HD - Data and Recovery. Yesterday I made another backup yesterday too and that backup showed:

Macintosh HD

Manctiosh HD Data

Macintosh HD-1

Macintosh HD-2

Recovery

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Answer 6

Kappy

Level 10

361,765 points

Feb 8, 2020 12:49 PM in response to ckralc

I've read your post three times and still can't figure out exactly what you are doing, why you are doing it, and what drive is the problem.

What drive are you booted from? There is no such thing as a single Fusion drive. A Fusion drive consists of a hard drive and an SSD. The latter is on the motherboard. Where and what is the hidden partition? And, what has any of this to do with Time Machine?

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Answer 7

Kappy

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361,765 points

Feb 8, 2020 2:46 PM in response to ckralc

Was this drive you connected as a Time Machine backup drive erased before you used it or did it have other backups on it? How was the backup drive formatted? Was it APFS or Mac OS Extended, Journaled (HFS+)?

Where and what is Macintosh HD-1. Is this a volume on the Fusion drive or is it the SSD part of the Fusion drive? If you have a 1TB Fusion drive, then the SSD is just large enough for macOS. It won't have other files on it. But if you have the larger SSD, then you might see your files on it. Of course, it will look to you that it is on Macintosh HD rather than the invisible SSD.

I assume your Fusion drive is formatted APFS given what you have shown above. But the Fusion drive is really a HDD joined to an SSD. The Fusion drive should be formatted Mac OS Extended, Journaled instead of APFS. APFS actually causes a HDD to function slower. APFS is optimized for use on an SSD, not on a HDD.

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Answer 8

ckralc Author

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5 points

Feb 8, 2020 4:58 PM in response to HWTech

I have the DriveDX report - I am not sure if I captured both the SSD and Hard Drive. I believe Apple is doing maintenance - it will not let me post the report in Additional Text. I will try again tomorrow.

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Answer 9

ckralc Author

Level 1

5 points

Feb 9, 2020 5:22 PM in response to HWTech

HWTech - thank you for the feedback. I thought I had erased the base of the Fusion Drive in the past, but it is proving difficult to do at last try. Thank you for the info on the Time Machine (and Kappy's) - I hadn't realized that a TM should not contain backups of other computers - I hadn't realized it can cause confusion. I think what you may be saying is that my data - what I thought was data NOT ERASED - may well have been erased but TIme Machne was confused and carried it over from previous backups?

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Answer 10

HWTech

Level 9

56,486 points

Feb 9, 2020 5:50 PM in response to ckralc

Yes, that is what I (we) think could be happening.

In your screenshots of Disk Utility you want to select top most item labeled "Fusion Drive" as that should be the base. I've only worked with a Fusion Drive just once or twice and didn't really take notice of the little things. I thought Disk Utility would still display the physical drives of the Fusion Drive, but perhaps I'm remembering incorrectly or perhaps later versions of macOS/Disk Utility have changed it and may only show the Fusion Drive so users do not get confused with all the drives.

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