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Note: I am not looking for a recommendation! I am looking for a reason why M.2 and 2.5 disks are not supported at the same time.

I am able to find only a few laptops that support M.2 and 2.5 disks at the same time so I can have 2 disks in my laptop and switch the booting order in the BIOS as I wish to load different OSs.

Is there a reason for not supporting M.2 and 2.5 disks at the same time in laptops?

Btw. Here is the list http://laptopmedia.com/laptop-m-2-ngff-ssd-compatibility-list/ and as you can see only a tiny fraction support M.2 as a disk with 2.5 disk at the same time.

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Same thing happens on desktops. An M.2 slot relies on PCIe and SATA lanes from the chipset/PCH (Platform Controller Hub) which are limited in number. An example of a block diagram for one is here.

MB manufactures then have to (are allowed to) choose how they would like to distribute these lanes, some here some there, divided between SATA, T-bolt, USB3, Ethernet, and whatever else loves to have the speedy more direct communication lanes.

On many desktops the M.2 choice disables a couple of the SATA, for example, because they share lanes. The MB manufacture chooses where to distribute them, the user gets the choice of losing one for the other or visa versa. Some other manufacture might choose to knock out a few lanes for the other gpu slots.

Different CPU architextures (the cpu chip itself) have different quantities of lanes avialable. to further your question, the actual CPU that is in use would be interesting.

So that is the explaination you desired, the problem still is, there should be enough lanes available to cover these two items easily . Please provide all your specs, model cpu info in the question.

Other notes: Hubing the lanes out of the cpu is not without its own problems, there is minor slowdowns via the negotiation, and probably buffers used in the chips that set up for sharing of these direct lanes, which are of course no longer so direct.

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Laptops support 2.5" drives and M.2 drives at the same time just fine. Almost all support them simultaneously.

Psychogeek's answer explains why there are sometimes limitations but both that and you are missing the main point..

Btw. Here is the list http://laptopmedia.com/laptop-m-2-ngff-ssd-compatibility-list/ and as you can see only a tiny fraction support M.2 as a disk with 2.5 disk at the same time.

No no no no no. That page has nothing to do with 2.5" disks at all. It's purely about listing M.2 slots. M.2 slots by definition are designed to be smaller than previous generation slots so they are frequently used in smaller laptops that don't have space for a 2.5" slot anyway. The ones that do, usually have both.

The HP EliteBook 820 has two M.2 slots. a mPCIe slot and a 2.5" slot. The Dell Inspiron 7000 has both M.2 slots and 2.5" slots. The Aorus X3 v5 has multiple M.2 slots, because it's too thin for 2.5" slots. The V3/V4 have three mSATA slots for the same reason. The 14"+ Lenovo Thinkpads all have both M.2 slots and 2.5" slots. In fact, any current-gen laptop that has WiFi has an M.2 slot - because an M.2 slot is required to house current-gen WiFi cards.

tl;dr: Many larger laptops support M.2 and 2.5 disks at the same time

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