Boot partition contains necessary files, which you need to boot the OS. You can find boot partition easily by using Disk Management.
Connect your external hard drive to your computer and then follow the steps given below:
Step 1: Go to Control Panel - System and Security - Administrative Tools - Computer Management - Disk Management.
Step 2: In the Status column, boot partition must be marked as Boot.
Excluding this, if you want to make any other partition as an active bootable partition, then follow the guide mentioned below.
Step 3: Go to Windows Start button and type cmd in the search box.
Step 4: It shows you shortcut for Command Prompt. From the context menu, select Run as Administrator.
Step 5: In the Command Prompt window, type diskpart and enter Return.
Step 5: It will launch the Diskpart prompt. Here you need to enter list disk command.
Now, it shows you the list of hard drives. Write down the number assigned to drive that has a partition which you want to make boot partition
Step 5: Then, type select disk * (* indicates the number assigned to volume) and then hit Enter button.
Step 5: Now, type list partition command and press Return key. It lists all the partition exist on the drive.
Note down the number assigned to partition, which you want to make bootable.
Step 5: Then, type select partition *, replacing “*” (*-indicates the number assigned to partition) and then hit Enter.
Step 5: Type Active and click Return key. It shows to Diskpart that the partition is active bootable partition.
Step 6: Finally, type Exit and then Return. Close the Command Prompt.
Now, go to disk management and check for a bootable partition. In case, while creating a bootable partition, if you get into any mishaps which results in data loss, then you need to make use of Remo Partition Recovery software. The software also comes handy to restore data from formatted partition as well as corrupted partition which if formatted with FAT, NTFS, exFAT and HFS file systems.