Partitioning a drive divides it into separate, discrete sections called partitions. This allows you to have multiple partitions on a single physical disk that function as if they were separate disks. When you partition a drive that already contains data, what happens to that existing data depends on the partitioning scheme and methods used. There are a few key things that can occur:
- The existing data may be left untouched and remain accessible in its original partition.
- The existing data may be erased or overwritten when the new partitions are created.
- The existing data may be redistributed between the new partitions.
The outcome depends on choices made when initializing the new partition table and boundaries, whether the data is moved/copied to new locations, and actions taken to preserve or remove data from the original partition(s). Understanding these factors can help avoid unintended data loss when repartitioning a disk.
What is Disk Partitioning?
Disk partitioning is the act of dividing a physical data storage device, like a hard disk drive or SSD, into multiple logical storage units called partitions. Partitions behave like physically separate disks, with their own filesystems, operating systems, and data. However, they share the underlying physical disk.
Some key aspects of disk partitioning:
- Partitions are created by writing partition tables and boundaries to the disk.
- These define the partitions’ locations and sizes on the disk.
- The disk can then be logically divided into separate partitions.
- Each partition can be formatted with a filesystem (NTFS, FAT32, etc.) to organize data storage.
- Partitions can have mounted drive letters and house operating systems or data.
- The disk remains physically one unit but is split at the software level.
Partitioning is commonly done to:
- Logically separate different operating systems (dual boot).
- Separate system files from user data.
- Isolate sensitive data.
- Divide a large disk into smaller logical units.
The basic partitioning process involves:
- Creating partition layout with sizes and types.
- Writing this layout to the partition table on disk.
- Creating filesystems on the new partitions.
- Assigning drive letters/mount points to partitions.
This initializes the divisions on disk so they can be used by operating systems and applications.
What Happens to Existing Data When Partitioning?
When partitioning a disk that already contains data, that original data is affected in some way by the process. The exact impact depends on:
- The current file system and partitions.
- The new partitioning scheme and layout.
- The partitioning tools and methods used.
- Whether data is moved/copied to new partitions.
- Steps taken to remove or preserve original data.
Some potential outcomes include:
Data is left untouched in original partition
If the new partition boundaries are created in free space on the disk, the original partition(s) and data may be left completely untouched and remain accessible.
This preserves the data in its current state. However, if you need to resize partitions, move data to new partitions, or change file systems, the data will have to be modified or copied.
Data is erased/overwritten
Some partitioning tools automatically erase all existing partitions and data when initializing a new partition table. This destroys all original data.
Creating new partitions in allocated space also overwrites original data in those areas. Any in-use sectors that overlap with new partitions will be overwritten and data will be erased.
Data is redistributed among new partitions
In some cases, existing data may be intelligently redistributed among the new partition layout based on space, usage, file types, etc.
For example, personal user files from an old home partition may be moved to a new “data” partition, while applications stay in the new “system” partition.
This can preserve data while migrating it to appropriate partitions. However, data is still being moved/changed from its original location and partitions.
Preserving Data When Partitioning
If you want to prevent data loss when repartitioning, there are some best practices to follow:
- Backup important data before making any partitioning changes.
- Understand partitioning tools and capabilities up front.
- Leave existing partitions/data alone if possible by creating new partitions only in free space.
- Use partitioning tools that move data intelligently to new partitions as needed.
- Manually move data to different partitions or storage devices before repartitioning.
- Delete unneeded data and resize existing partitions first to free up space.
Always assume data outside the new partition boundaries may be inaccessible or erased unless steps are taken to preserve it. Be prepared to restore from backups if needed.
Examples of Data Loss During Partitioning
To illustrate some of the risks, here are a few examples of what can happen to original data when repartitioning disks:
Initializing a blank new partition table
If you create a completely new blank partition table, this will erase all previous partitions and filesystem data. For instance, using DiskPart’s “clean” command in Windows.
All data in the old partitions would be overwritten and permanently lost.
Creating overlapping partitions
Say you shrink an existing 100GB C: partition to 60GB to make room for a new 40GB D: partition.
The new D: partition will overwrite and erase any data that existed in that overlapping 40GB portion of the old C: partition.
Resizing partitions with unmovable files
Trying to shrink a partition with unmovable system files in the area to be reduced may fail or introduce filesystem errors that make data inaccessible.
For example, you can’t safely shrink C: if the Windows system files can’t be moved out of the shrinkage area first.
In-use partition is deleted
Deleting a partition currently holding the active operating system or data files needed by open applications can lead to data corruption or access issues.
For instance, deleting C: while Windows is running from that partition.
Partitioning Tools & Methods
The partitioning tool and techniques used can greatly impact what happens to original data. Some tools are more intelligent and allow greater control than others.
Operating System Tools
Most operating systems like Windows, Linux, and macOS include built-in partitioning utilities that provide basic capabilities:
- Create, delete, format partitions.
- Set partition types, flags, drive letters.
- Move/resize some partitions.
But OS tools have limited ability to intelligently move data between partitions. And partition operations may be restricted while the OS is running.
Third-Party Partition Managers
Third-party partitioning tools like EaseUS Partition Manager, MiniTool Partition Wizard, and AOMEI Partition Assistant provide advanced capabilities:
- Non-destructive partitioning even of in-use disks.
- Intelligently move data and files between partitions.
- Migrate OS and convert filesystems.
- Resize partitions while preserving data.
They provide greater control over moving data to new partitions during repartitioning.
Disk imaging tools like Clonezilla allow capturing a full partition image backup before partitioning. This image can be restored to recreate the original partitions and data.
Imaging preserves all data intact before changes are made. The new partition layout is then applied empty.
Manual Data Copying
Manually copying all required data and files to another disk or partition before repartitioning is the safest approach.
Once the data is secured elsewhere, the disk can be freely repartitioned as all original data has been removed.
Thisrequires more work but provides full control over data.
Key Considerations Before Repartitioning
Follow these recommendations when planning to partition a disk with existing data:
- Have backups – In case anything goes wrong, make backups of important data.
- Understand your tools – Know how your partitioning tools handle data during operations.
- Plan partition layout – Design your new partition structure and sizes beforehand.
- Determine data handling – Decide how to handle moving, preserving, or removing existing data.
- Delete unnecessary data – Remove any unneeded data and files to free space.
- Consider imaging – Use disk imaging to preserve disk state before partitioning.
- Move data manually – As a precaution, copy important data to another location first.
Thinking through these factors will help avoid surprise data loss when repartitioning disks.
When partitioning a disk that contains existing data, that original data is at risk during the process. But with proper precautions, the impact can be minimized or avoided completely:
- Back up data and have a contingency plan for recovery.
- Use partitioning tools judiciously based on capabilities.
- Leave data untouched in original partitions if possible.
- Leverage capabilities to move data to new partitions intelligently.
- Manually copy important data to other locations before partitioning.
- Use disk imaging to preserve the original disk state if needed.
With careful planning and the right tools, you can partition disks while keeping valued existing data intact and accessible. Know the implications before repartitioning, and take steps to protect against unintended data loss.