Before installing Linux beside Windows, verify four things: your important data is backed up to separate media, BitLocker will not block the installer, Linux will boot in the same firmware mode as Windows, and the installer clearly identifies the correct drive and partition changes. If any proposed erase, resize, or target-disk action is unclear, stop rather than guessing.
Use this stop-or-go checklist first
A dual-boot installation changes partitions and boot information on a computer that already contains a working operating system. The safest preparation is not a particular partition size or a promise that a guided installer will preserve everything; it is a set of checks that makes the planned changes understandable before you approve them.
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- Backup: Copy important documents, photos, passwords, and other irreplaceable files to separate external media. Open a few files from the backup so you know the copy is usable.
- Windows recovery: Have a way to reinstall or repair Windows if the computer cannot start afterward. Ubuntu’s community dual-boot guidance recommends recovery media, but that page is marked as needing updates; use current Microsoft instructions for your Windows edition and device.
- Encryption: Determine whether BitLocker protects the Windows installation. Ubuntu’s installer cannot safely map an encrypted Windows installation for an alongside setup.
- Firmware mode: Establish whether Windows starts in UEFI mode or legacy BIOS mode, then boot the Linux installation media compatibly.
- Disk and space: Identify the physical drive that contains Windows, check that it has enough usable space, and confirm that the installer proposes the drive and allocation you intended.
- Installer wording: Read every erase, resize, and partition confirmation. If you cannot explain what will be changed, do not click Continue.
These checks apply broadly, but installer behavior differs by distribution. The specific alongside and BitLocker behavior described below is for Ubuntu Desktop. Other distributions may use different screens and have different Secure Boot support.
1. Make a separate backup before touching partitions
Back up files, not just the Windows installation
Partition resizing is a disk operation, so a guided “install alongside” option is not a substitute for a backup. Copy the files you would struggle to replace to an external backup drive or other separate storage before starting. Keep that backup disconnected while you install when practical; this reduces the chance of selecting it accidentally or damaging both copies in one operation.
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Prioritize material that may not be recoverable from an online account: personal photos and videos, tax and work files, browser exports, local email archives, encryption keys, and application projects. Confirm that the backup opens on another computer or in a separate file browser. A backup that was never tested is only an assumption.
Prepare Windows recovery options
A recovery drive or current Windows installation media can help repair startup problems or reinstall Windows if a disk change goes wrong. Ubuntu’s WindowsDualBoot guidance mentions having Windows recovery media, but the page itself warns that it needs updating. Follow Microsoft’s current recovery-media instructions for your exact Windows version rather than relying on an old menu path. Store the recovery media separately from the file backup.
If BitLocker is enabled, make sure you can retrieve the Windows recovery key before changing anything. Save it somewhere you can access without relying on the Windows installation you are about to modify. Do not publish or share the key as part of a troubleshooting post.
2. Check BitLocker and other Windows encryption
Why encryption can block an alongside installation
Ubuntu’s official BitLocker installation guidance says the encrypted contents are inaccessible to the installer. Without reading the Windows volume, Ubuntu cannot correctly map the existing installation and cannot safely offer the normal alongside layout. If the installer reports that BitLocker prevents the operation, treat that as a safety stop, not as a warning to ignore.
Decide how to handle BitLocker deliberately
Ubuntu documents turning BitLocker off as one option. Do not disable encryption casually: understand what protection you are removing, retain the recovery information, and check the current Windows instructions for your edition and device. Ubuntu also cautions that some Windows versions may not allow BitLocker to be turned on again after it has been disabled. That possibility should be part of your decision, especially on a work-managed computer or a laptop subject to an organization’s security policy.
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Before changing encryption, ask:
- Is this your personal computer, or does an employer or school control its encryption policy?
- Do you have the recovery key and know how you would use it if Windows asks for it after a firmware or boot change?
- Have you confirmed from current Windows documentation what “turn off” does on your version, how long decryption may take, and what state the drive will be in when you restart?
- Would installing Linux on a separate, unused drive meet your goal without altering the encrypted Windows volume?
Do not proceed until the answer to the relevant questions is clear. If the installer sees only an encrypted Windows volume and offers to erase it, that is not an alongside installation.
3. Match the firmware boot mode
Check how Windows currently boots
A computer can start an operating system through modern UEFI firmware or through a legacy BIOS compatibility mode. For a reliable dual-boot arrangement, Linux installation media should be launched in the same mode Windows already uses. Check the firmware setup and Windows system information for the current mode, then note which boot entry you select for the USB media. Some firmware shows two entries for the same USB device; one is UEFI and the other is legacy.
Ubuntu’s UEFI guidance addresses installing alongside a UEFI Windows installation, but that community page includes dated legacy instructions. Use it as background, not as a reason to apply old settings blindly.
Do not disable Secure Boot by default
Secure Boot support depends on the distribution, its current release, and the computer’s firmware. Verify the chosen distribution’s current official support statement and read the device manufacturer’s firmware guidance before changing the setting. Disabling Secure Boot is not a universal prerequisite for installing Linux, and changing it can trigger a BitLocker recovery request or a policy warning on some Windows systems.
Record the settings before changing them
Take a photograph or written note of the current boot order and security settings. If you change a setting temporarily to test the installation media, you will have a reference for restoring it. Never change several firmware options at once; otherwise you will not know which change caused a new startup behavior.
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4. Confirm the drive and available space
Identify the physical target
Many computers contain more than one internal drive, and Windows may place its boot files on a different drive from the main Windows partition. Read the model, capacity, and partition layout shown by the installer rather than relying on labels such as “Disk 0.” Compare those details with the drive information you recorded in Windows. An external backup drive can also appear in the list, so unplug nonessential storage before launching the installer.
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Understand Ubuntu’s alongside flow
In the current Ubuntu Desktop installation flow, the alongside option lets you select the drive and choose how much space Ubuntu will use. The amount available is limited by the space occupied by existing files. The screen is a proposal, not a guarantee that every displayed choice is harmless: inspect the selected drive, the Windows partition, the Linux allocation, and any stated changes before accepting.
There is no universal Linux partition size that is safe for every reader. Your decision depends on how much software, updates, containers, virtual machines, and personal data Linux will hold, as well as how much free space Windows needs to operate. Leave Windows enough working room for its updates and applications, and do not treat the largest number the slider permits as the correct number.
Stop when the proposed layout is unclear
Look for a plain-language description of which partition will shrink, which space will become available to Ubuntu, and which drive will receive the boot files. If the installer shows an unexpected disk, an implausible Windows size, or a warning you cannot interpret, cancel and investigate from a live session or another documented path. Do not use trial-and-error clicks on a disk containing your only Windows copy.
5. Know what each installation path means
Ubuntu presents different disk-setup choices. Their consequences are not interchangeable.
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| Path | What it is for | Checks before proceeding |
|---|---|---|
| Install alongside | Uses the existing drive, asks which drive to use, and lets you choose the amount of space Ubuntu should occupy. | BitLocker must not prevent mapping; the selected physical drive must be the intended one; the proposed resize and allocation must be understandable. |
| Erase disk | Replaces the selected disk’s existing contents with the Linux installation. | Use only when you have deliberately chosen that disk and have a verified backup. Ubuntu’s BitLocker documentation states: “This is a destructive operation, with no option to recover any Windows data.” |
| Manual partitioning | Lets an experienced user define partitions and mount points instead of accepting an automated layout. | Know exactly which partition is Windows, which is the EFI system partition, where Linux will be installed, and how boot files will be handled. Ubuntu describes this as an advanced-user workflow. |
Ubuntu’s official installation page gives a useful rule for confusing screens: “If the interface doesn’t make sense, we recommend that you pick one of the automated disk setup options instead.” That does not mean accepting an automated option without reading it; it means avoiding manual partitioning when you cannot confidently predict the result.
6. Prepare installation media safely
Create bootable media for the distribution you selected
Ubuntu’s installation process uses bootable installation media. A USB flash drive is a practical choice for a USB-based workflow. The material available here does not establish a universal capacity, model, or retailer, so choose media that is known to work with your computer and follow the distribution’s current instructions for writing the image.
Writing an image normally removes the existing contents of the USB drive. Copy anything important off that drive first. Keep the USB clearly labeled so you do not confuse it with a backup disk or a Windows recovery drive.
Test the boot path before changing the disk
Boot from the installation media and verify that the live environment can start, recognize the keyboard and display, and show the internal drive you expect. Recheck the firmware mode shown by the boot menu if the installer behaves differently from the documentation. Reaching the installer is not permission to continue; it is an opportunity to confirm that the media and firmware choices are correct.
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- Finish the backup. Close files, copy them to separate media, test several copies, and disconnect the backup if you can.
- Secure recovery information. Confirm access to Windows recovery media and the BitLocker recovery key where applicable.
- Check encryption status. If BitLocker is enabled, decide whether to keep it and use another installation design, or follow current Windows guidance for a deliberate change.
- Document firmware settings. Record UEFI or legacy mode, boot order, and Secure Boot state before altering anything.
- Reduce drive confusion. Disconnect nonessential external drives and identify the internal drive by model and capacity.
- Boot the installer compatibly. Select the installation-media entry that matches Windows’ boot mode.
- Read the disk proposal. Confirm the target drive, Windows partition, amount of space, and every listed operation.
- Commit only after a second review. If a warning, label, or size does not make sense, cancel instead of assuming the installer will choose correctly.
8. Recognize common stop conditions
The alongside option is missing
BitLocker, an unsupported layout, insufficient usable space, or a boot-mode mismatch can prevent Ubuntu from offering an alongside choice. Do not substitute “erase disk” simply because it is available. Return to the encryption, firmware, and drive checks and consult the current Ubuntu documentation for the specific message.
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The installer sees the wrong disk
Stop if the model or capacity does not match the drive you intended. Shut down, remove external storage, and recheck the firmware and Windows disk information. A label that looks familiar is not enough evidence to approve a destructive change.
The space proposal is smaller or different than expected
Ubuntu calculates available allocation around existing files. A nearly full Windows partition may offer less space than its nominal capacity, and files that cannot be moved easily can affect a resize. Do not force the slider or move to manual partitioning unless you understand the layout and the consequences.
The interface or warning is confusing
Take a screenshot or write down the exact wording, then stop. Ubuntu explicitly recommends an automated option when its interface does not make sense; it does not recommend guessing at manual partition fields. Resolve the ambiguity using current official documentation for your Ubuntu release and hardware.
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Use the recovery information you saved before installation. Do not delete partitions or repeatedly change firmware settings while trying random fixes. Once Windows is accessible, review its encryption and boot settings before attempting another Linux installation.
9. Final approval questions
Right before you approve the disk operation, answer these questions in plain language:
- Where is my tested backup, and can I access the Windows recovery key?
- Is BitLocker enabled, and have I made an informed decision about keeping or changing it?
- Am I booted in the same firmware mode Windows uses?
- Can I name the exact physical drive that will change?
- How much space will Ubuntu use, and what Windows partition will be resized?
- Does the installer clearly distinguish alongside installation from erase-disk installation?
- Can I explain every proposed change without relying on a guess?
A “no” to any question is a reason to stop and resolve that item first. Once the backup, encryption, firmware, target-drive, and proposed-layout checks all pass, you can make an informed decision about continuing with Ubuntu’s installer rather than treating dual boot as a risk-free automated operation.
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