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I have 2 disks (disk 1 and 2), both are exactly 2794.52 GB.
Disk 2 is formatted and contains important data,
Disk 1 is unallocated.

Note that I cannot back up my data and format Disk 2 due to the sheer size of the data it holds.

When attempting to mirror Disk 2 to Disk 1 using Windows Disk Management, it tells me that I do not have enough space. Which is odd since they're off the same size, and Disk 2 is only 40% full.

They are both a Type basic, since the Disk Management is supposed to convert them to dynamic during mirror.

I have spent several hours searching for a solution but have not found one. Why do I get this error when they are both the same size and have enough space?

Screenshot: Disk Management

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    More of a topic for SuperUser, however Dynamic Disks were deprecated 10 years ago. Consider using Storage Spaces. You should also consider backing up the data. Mirrors are not useful for recovery. Windows 10 has the same backup application that Windows 7 had (sdclt).
    – Greg Askew
    Sep 30 at 9:53
  • @GregAskew I researched a bit about Storage Spaces and while it does do the same thing conceptually as RAID 1, it won't work for me since it'll format Disk 2, which as mentioned before I am unable to back up due to the sheer size of the data contained.
    – Ralkey
    Sep 30 at 10:07
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    Additionally, no third part disk/storage tools work with Dynamic partitions. It's a one way trip, with no backup. You should at least look into an online backup service.
    – Greg Askew
    Sep 30 at 10:15
  • RAID is no backup.
    – Zac67
    Sep 30 at 10:21
  • I don't need it for backup, I just need some redundancy so when one of my drives fail (which it will) all my data won't be lost. That's why I chose to go with RAID 1, which I currently can't get to work on my machine.
    – Ralkey
    Sep 30 at 10:24

2 Answers 2

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Note that I strongly advise against creating RAID on a disk for which you have no backup, since any error may cost you your data. I would rather advise copying the data and running regularly a sync job, which is much safer.

For your problem: The disks must be completely identical in all attributes. This can be checked using the command

fsutil fsinfo ntfsinfo X:

As you found that the MFT sizes are not equal for both disks, this means that the available file-space is not the same for both.

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  • Slight clarification. Files that are 1kb or less are stored directly in the MFT... traditional Mirrors seems like a known limitation or something to avoid if it can manifest in this symptom. I would expect this to not be an issue unless there has been a lot of creation of small files or directories, which are zero byte files. Maybe a defrag of the source disk to shrink the MFT if there is a lot of slack space. learn.microsoft.com/en-us/troubleshoot/windows-server/…
    – Greg Askew
    Sep 30 at 13:05
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RAID is not a backup.
RAID is not a replacement for backup.
RAID is not an alternative strategy to backup.

RAID is a tool for improving your availability, i.e. uptime. It allows some types of hardware failures to be mitigated until the failed drive can be replaced.

However, more than half of data loss incidents are caused by human and software error, and RAID does absolutely nothing against that. Data gets deleted or corrupted on both drives at once. RAID is more complicated than a single drive and increases the risks of software or human error.

Using RAID 1 without a backup only makes sense when the data is of zero value. An example is the contents of a game console drive, which can be recovered from the server at any time. In other cases, create a backup first, think about RAID second.

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    If you refuse to make any attempt to answer the question, then have the decency not to click the "answer" button.
    – hobbs
    Sep 30 at 20:12
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    And please actually read the post, I never said I wanted to use RAID for backup, I know what RAID is used for. I even clarified it here
    – Ralkey
    Sep 30 at 20:40

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