Fdisk will not see a NTFS partition, so you could only mark a Fat partition
as active.
From Windows help:
To mark a partition as active
Using the Windows interface
Open Computer Management (Local).
In the console tree, click Disk Management.
Where?
Computer Management (Local)
Storage
Disk Management
Right-click the primary partition you want to mark as active, and then click
Mark Partition as Active.
Notes
To open Computer Management, click Start, and then click Control Panel.
Click Performance and Maintenance, click Administrative Tools, and then
double-click Computer Management.
You must be logged on as a computer administrator in order to complete this
procedure.
In the list and graphical views, the system, active, and boot partitions are
marked as System, Active, or Boot, respectively.
You cannot mark a logical drive as active.
You can only mark a partition as active on a master boot record (MBR) disk.
A computer can only have one active partition per disk.
Changing or deleting an active partition might cause your computer not to
start.
The names commonly used for partitions containing the startup and operating
system files are system and boot partitions, respectively.
The system partition must be a primary partition that has been marked as
active for startup purposes and must be located on the disk that the
computer accesses when starting up the system. There can be only one active
system partition on a disk at a time. You can have multiple basic disks and
each can have one active partition. However the computer will only start
from one specific disk. If you want to use another operating system, you
must first mark its system partition as active before restarting the
computer.
Using a command line
Open Command Prompt.
Type:
diskpart
At the DISKPART prompt, type:
list partition
Make note of the number of the partition that you want to mark as active.
At the DISKPART prompt, type:
select partition n
Select the partition, n, you want to mark as active.
At the DISKPART prompt, type:
active
Important
DiskPart verifies only that the partition is capable of containing an
operating system's startup files. DiskPart does not check the contents of
the partition. If you mistakenly mark a partition as "active" and it does
not contain the operating system's startup files, your computer might not
start.
ValueDescription
list partitionDisplays the partitions listed in the partition table of the
current disk.
select partitionSelects the specified partition and gives it focus. If no
partition is specified, the select command lists the current partition with
focus.
activeOn basic disks, marks the partition with focus as active. This informs
the basic input/output system (BIOS) that the partition or volume is a valid
system partition or system volume. Only partitions can be marked as active.
Notes
To open command prompt, click Start, point to All Programs, point to
Accessories, and then click Command Prompt.
You must be logged on as a computer administrator in order to complete this
procedure.
You cannot mark a logical drive as active.
You can only mark a partition as active on a master boot record (MBR) disk.
A computer can only have one active partition per disk.
Changing or deleting an active partition might cause your computer not to
start.
The names commonly used for partitions containing the startup and operating
system files are system and boot partitions, respectively.
The system partition must be a primary partition that has been marked as
active for startup purposes and must be located on the disk that the
computer accesses when starting up the system. There can be only one active
system partition on a disk at a time. You can have multiple basic disks and
each can have one active partition. However the computer will only start
from one specific disk. If you want to use another operating system, you
must first mark its system partition as active before restarting the
computer.
