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The following guide documents BootGuard error BG-4PP and provides a resolution to the boot-time error screen presenting this problem.
<h3>BootGuard Error BG-4PP</h3>
When booting from a PC that has been protected by NeoSmart Technologies'<sup style="font-size: 60%;"> ® </sup> BootGuard™ technology on a PC that has been configured to start up in legacy/MBR/CSM mode, the following message with error code BG-4PP may be displayed:
<div style="background-color: black; padding-top: 2em; padding-left: 2em; margin-bottom: 2em;"><a href="https://neosmart.net/wiki/wp-content/uploads/sites/5/2017/02/bg-4pp.png"><img class="alignnone size-full wp-image-5235" src="https://neosmart.net/wiki/wp-content/uploads/sites/5/2017/02/bg-4pp.png" alt="" width="720" height="400" /></a></div>
<blockquote>BootGuard by NeoSmart Technologies

This PC has been prevented from booting in legacy mode to protect your data.
Please re-configure your UEFI firmware (aka "BIOS setup") to boot in UEFI mode.

Error Code: BG-4PP
More Information: http://neosmart.net/wiki/bootguard-4pp/

Press CTRL+ALT+DEL to restart</blockquote>
<h3>What is BootGuard™?</h3>
BootGuard™ is a boot-time helper that is installed to the MBR of your boot disk in order to protect you from accidental data loss due to a misconfigured PC. It is normally installed as part of an automated boot initialization or  repair procedure, such as our Easy Recovery Essentials<sup style="font-size: 60%;"> ®</sup> bootable Windows repair and recovery CDs. BootGuard is normally 100% invisible, and only kicks-in if it detects a problem with your boot configuration, preventing from using your computer in a way that might trigger data loss.
<h3>BootGuard error BG-4PP</h3>
The specific BootGuard error BG-4PP is shown when one or more of the disks in your PC are partitioned in such a way that they cannot be represented in a typical MBR. Traditional <a href="http://neosmart.net/wiki/mbr-boot-process/">BIOS/MBR PCs</a> use a partition management scheme that dates back to the early 1980s, and cannot handle more than 4 so-called "primary" partitions at any given time. A number of workarounds have been developed to address more than 3 partitions on a legacy disk, but they rely on the use of a concept known as "logical partitions" which require the availability of a certain amount of free space preceding each partition past the 3rd. Modern PCs can use what is known as a GPT disk format to handle up to 128 partitions without any such restrictions.

Older PCs cannot normally boot from a GPT disk at all; EasyRE enables your PC to boot in both traditional BIOS/MBR mode or in UEFI/GPT mode so long as it is able to create a logical partition structure on your disks that coexists with the GPT layout used for UEFI boot. However, if you try to boot into a system that needs more than 4 primary partitions to properly represent its structure, the BootGuard error BG-4PP error is shown to prevent your PC from data loss.
<h3>What happens if you boot in MBR mode bypassing this error?</h3>
Without BootGuard and with your PC set to boot in legacy/MBR mode, a disk will be limited to only four partitions when booted in legacy mode. Moreover, traditionally a "protective EFI partition" is created to prevent the overwriting of the GPT partition table when booted in MBR mode, effectively reducing the maximum number of supported partitions to only three. In the best case scenario, you'll only be able to use the first 3 of your partitions - but this is unfortunately rarely what actually happens.

In reality, because BIOS/MBR and EFI/GPT modes are completely separate, what typically happens is that after a while the MBR and GPT begin to differ in their representation of the partition layout on the disk. One might presume free space is available on the drive where the other expects to find pre-existing partition data, and permanent disk corruption and data loss is a real concern! BootGuard intercepts attempts to boot the PC under such conditions, and reminds you to re-configure your PC to boot in UEFI mode instead with the BG-4PP error.
<h3>How to fix BootGuard error BG-4PP</h3>
The solution is quite simple: you need to enter your firmware setup/configuration utility (more commonly, if incorrectly, known as BIOS setup) and set up your PC to boot in UEFI mode. We have published detailed instructions on how you can <a href="https://neosmart.net/wiki/enable-uefi-boot/">disable legacy boot and make sure your PC is booting in UEFI mode</a> on your UEFI-capable PC in our knowledgebase.

Make sure that if your PC supports both UEFI and Legacy/CSM boot modes simultaneously that it is configured to prioritize UEFI booting.

Our <a href="http://neosmart.net/EasyRE/">Easy Recovery Essentials</a> bootable repair & recovery CD can also address this problem by attempting to create a logical partition structure on your disk that mirrors the GPT layout, but this can only succeed if there is free space preceding each of the additional partitions past the 3rd primary partition in order to hold the logical partition structures. Without this space, EasyRE will install the BootGuard boot-time data protection helper, and the BG-4PP error will persist.