Can I install Ubuntu on a clean harddrive and keep rest as unallocated?

Question

So i have a fresh new 1TB hdd and I don't want to install windows now because i don't have installation key. I want to install linux on a 200gb partition and leave rest as unallocated space for Windows installation Is there any harm keeping space unallocated

Whenever I install windows, will i still be able to use GRUB or windows analogue of it?

I want my pc to boot windows by default how do I do that?

Answer 1

Install Windows without License

You do not need the key to install Windows, See https://www.microsoft.com/en-us/software-download/windows10startfresh

However Windows still technically requires a license for full performance.

Without activating, you won't be able to personalize the desktop background, window title bar, taskbar, and Start color, change the theme, customize Start, taskbar, and lock screen etc.

Additionally, you might periodically get messages asking to activate your copy of Windows

It might make your life easier to install Windows 10 before installing Ubuntu.

You can let Windows sit until you get a key and then activate it when you are ready.

Windows is a little bit useful even without activation, you can always delete it is you decide Ubuntu is enough.

Answer 2

Yes you can do this.

Install Ubuntu to a 200GB partition, but leave the rest as unallocated space. This is absolutely imperative because the Windows installer will NOT install to an already formatted partition, you need unallocated space to make it's own partitions.

You can also keep using GRUB too. Windows may install it's boot manager to the EFI partition that GRUB makes, or it may make it's own. You need to keep both either way, however you can re-order your BIOS entries so you can boot into GRUB. Once installing Windows and it's boot manager, you need to regenerate the GRUB boot loader image otherwise it may not show up with the Windows Boot Manager option.

Answer 3

What you're asking here is both trivial and needs attention to specific details in order as to not invalidate the "install Windows later" plan.

Firstly, this answer deals with only one detail of your question that was, after all, just a side remark. However tangential as it may seem, it touches on very important information the casual users are seldom familiar with. Namely it mentions that no product key is needed to install Windows 10. Also of note is oldfred's comment under your question:

If system was originally Windows the product key is in UEFI and will be reused with new install.

So, in a nutshell, please know that you can install Windows first if you're more comfortable with the approach and if the computer has had Windows already installed and is UEFI then you don't need to enter the product key again or ever.

The other answer starts by answering the gist of your question correctly. But then uses wrong terminology - and the distinctions that answer refuses to make can potentially ruin the whole plan -, wrong facts and incorrect suggestions.

To start with something there are already canonical answers to this hugely popular topic:

How can I install Windows after I've installed Ubuntu?

And specifically for UEFI: this answer

So, in summary, if you opt for installing Ubuntu now and later install Windows 10, here's what you absolutely need to know -and- plan ahead:

1. Know your computer, first and foremost; you need to know whether its firmware is the 40 years old BIOS or the new UEFI. Windows has strict requirements regarding partitioning - MBR ("msdos" or "dos") for BIOS installations and GPT for UEFI installations - therefore you must prepare your drive(s) accordingly beforehand (Ubuntu has no such requirements but it's always better to have GPT unless Windows in BIOS mode is in the equation)
2. Prepare your target drive accordingly the follow-up of #1; use Gparted in the live session, before starting the actual installation, select your drive and then use the Devices menu > Create partition table... and select accordingly
3. Install Ubuntu preferably using Something Else so you can set size of ESP, / (root). and /home, if desired, leaving the space intended for Windows unallocated, not because Windows can't be installed in an already formated partition (it CAN be installed in any NTFS formatted partition) but because it's preferable and easier to just leave blank space that the other OS installer will use later and partition as it wants.
4. If BIOS you MUST use MBR and you're strongly advised to install Windows first. With BIOS only one bootloader can be used per drive (at the MBR); installing Windows after Ubuntu will remove the Ubuntu bootloader and install Windows's and the result is an unbootable Ubuntu, situation that can be corrected - reinstalling Grub - but it's preferable to be avoided altogether (cf. next point)
5. Understand that the Windows bootloader can't be used for dual- or multi-booting except when only other Windows are involved (and regardless of BIOS or UEFI modes); the other answer incorrectly suggests - perhaps unintentionally - that Grub is optional; IT ISN'T; you may use a third and more complex to integrate rEFInd bootloader but you won't certainly be using Windows; there's really no need for added complexity for something that, again, is trivial
6. "Windows may install its boot manager to the EFI partition that GRUB makes, or it may make it's own" (from the other answer) is wrong on two counts; Windows installer necessarily, not optionally, installs its own bootloader in the ESP (EFI System Partition) and the ESP is not "made by Grub"; it's created by the installer or created by the user with manual partitioning as alluded to in #3
7. Knowing how to manage the boot order in the firmware IS imperative! If BIOS you need to assure the drive where the Grub has been installed has the first priority (it usually has, there should be no change from Windows only to the dual-boot with Grub the original Windows bootloader) but if UEFI you need to first assure the drive containing the ESP has the first priority and then change the "OS Selection" to "Ubuntu" (or back to... assuming that in this case Ubuntu has been installed first), two different settings that vary a lot depending on the manufacturer.
8. After installing Windows and setting the boot back to Ubuntu (Grub), it won't be "aware" of the new Windows and will boot Ubuntu directly; Open the terminal and run sudo update-grub