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I have two 500GB hard drives in my laptop. I have formatted both so they contain nothing. When I'm installing Windows 7, depending on the drive I choose to install Windows on I may or may not get the 100MB added.

When I install on drive 0 it doesn't get added.

When I install on drive 1 it always gets added.

Why would the partition be added on one drive or not the other?

I think it's the primary HDD that doesn't get the 100MB and the secondary one does.

PriestVallon
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    Its the other way around. As for the reason its because boot pritority. – Ramhound Feb 19 '13 at 12:23
  • So no matter which drive I install on, the 100MB should appear on the primary drive? – PriestVallon Feb 19 '13 at 12:35
  • I didn't say that. Change the boot order of the disks, verify my theory, then report back. – Ramhound Feb 19 '13 at 12:58
  • http://superuser.com/questions/330178/why-does-windows7-create-two-partitions – Oliver Salzburg Feb 19 '13 at 13:22
  • Would it be to do with which drive is the master and which is the slave? Or the boot order? I won't be on my laptop for a while to test out. – PriestVallon Feb 19 '13 at 13:22
  • @OliverSalzburg I know why it's there in general but I want to know why it would be there for one drive and not the other. – PriestVallon Feb 19 '13 at 13:25
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    @PriestVallon: I just wanted to leave the link for reference :) – Oliver Salzburg Feb 19 '13 at 13:26
  • Nothing to do with master or slave, and nothing to do with primary or secondary, these are terms used with which IDE/ATA port you plugged hard drives in!! And probably nothing to do with boot order. I don't think Ramhound is going to provide a source for what he has said. – barlop May 18 '14 at 16:42
  • The 100MB partition is the Active, Primary Partition. As can be seen in disk management. No doubt you can install Win7 to another drive it'll have 2 partitions, the 100MB one and the system one. I suppose it'll be active too. And you choose what to boot from in the BIOS boot order. But that won't affect what is installed (unless somebody wants to bring some good argument that it does!) – barlop May 18 '14 at 16:49
  • i am not sure that it'd even be boot order at the time that windows was installed, because boot order may be CD first. Maybe the drive on the SATA0 connector. – barlop Jan 08 '15 at 15:08

3 Answers3

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This “stub” of a partition, which is new in Windows 7, serves two functions. First, it holds the Boot Manager code and the Boot Configuration Database.
Second, it reserves space for the startup files required by the BitLocker Drive Encryption feature. If you ever decide to encrypt your system drive using BitLocker, you won’t have to repartition your system drive to make it possible.

Understand (and Get Rid of) the Mysterious Small Partition

I really don't understand why it wouldn't be created if you install to the first drive, unless you have a pre-existing partition on it. Usually, the partition is always created on an empty drive for the reasons quoted above.

Oliver Salzburg
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  • I've had it not create the partition when installing from a USB stick. I guess the USB stick was somehow picked up as the first drive. Caused all sorts of problems because it wouldn't create the partition but then later wouldn't boot (or some other problem, can't remember specifics) because the 100 MB partition wasn't there. – Kibbee Feb 19 '13 at 13:48
  • @Kibbee people install from usb stick often. I did onto a fresh drive and it made 2 partitions one 100MB one the system one. I haven't seen it do differently. Funny that your 100MB partition wasn't created. – barlop May 18 '14 at 16:45
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That partition must always be created mostly in the primary drive (master) since it contains the boot loader and all boot configuration stuff, but it is normally not created during normal disk-formatting on other drives/hardisk. It will be created if you format a drive or partition as drive as primary partition. As can be seen with diskpart's create partition primary <----- here goes your 100mb boot partition.

user3148991
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  • the terms primary and secondary master and slave are for IDE/ATA. I guess you mean the drive on the SATA0 connector? – barlop Jan 08 '15 at 15:09
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The 100 MB partition is used to store some boot info and as a store for essential files for Bitlocker encryption. There are complex ways to get rid of it, but in most situations it would not be worth the effort to free up 100MB.

Andrew Scott
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