Cannot boot back into Zorin OS after booting into Windows 10 once

[Aravisian]

@BptGrm

For now, as I understand it, the GRUB is located within my Zorin partition itself.

It places files in /boot and in the EFI partition.

[BptGrm]

Thank you for the very detailed intructions.

@337harvey

Choose the windows efi boot partition (100mb) and overwrite it

I assume that could theoretically pose a risk to Windows booting normally ?

@337harvey

Any other partitions that you want to mount, change them to their respective fs, no format and mount location

Again, I'm assuming that means the Windows partition ? I don't have a separated partition for data, but I have a few other that I don't really know about.
In total there's :

• a 260 mb FAT32 partition called SYSTEM
• a 128 mb of unknown (according to Windows) format and without any name
• the main NTFS C: partition named "Windows"
• a 694 mb NTFS partition without any name (I assume that's for Windows recovery ?)
• a 50 gb partition that I cleared for Zorin

[BptGrm]

Thanks again for the info !
Sorry for the delay, as this is a work computer there are sometimes long calculations that need to be performed, and in those cases I cannot shut it down for up to several days in a row.

I started following your instructions, and I have just a couple additional questions :

• First, there's a partition refered to as Windows Boot Manager, and it is slightly larger than what you mentioned as "Windows Boot Partition", at 260 Mb instead of 100 Mb. I'm pretty sure it's the correct one, but I'd rather not overwrite something I'm not supposed to I ask especially because there's another partition that do more or less match the size you described, at 128 Mb, but there's no indication on what it might be.
• Second, I do not have space cleared for a swap partition. I've heard it mentioned often during Linux installs, but since this computer shipped with Windows installed and has 32 Gb of RAM, I wonder if I have to have a partition that large in order for it to run properly.

EDIT : just to be clear, I can make space for a swap partition if needed, I just didn't plan for it during my first install.

[BptGrm]

Well I'm slightly more confused now.

To be clear, I should create a 500 Mb partition as "EFI system partition", which I pick as a boot loader location, and which should be at the top of my boot order after the install.

Then I should pick the existing Zorin ext4 partition, and choose to mount is as "/".

@337harvey

and check the format button.

But shouldn't I actually make sure it isn't checked if I want to recover that partition ?

And as I understand it, the default installer places both the bootloader and / on the same partition ?

Thanks !

[BptGrm]

Well, it worked !

I did get a warning after the install was completed that some of my programs couldn't be saved, and indeed the install reverted back to default apps, but other settings and all my files were recovered.

I cycled between Windows and Zorin a couple times to make sure that I could safely boot with both OSes and it looked fine, hopefully Windows won't break anything in the future.

Thank you everyone for the very extensive help, especially @337harvey and @Aravisian. I'll try everything out on my work laptop for a couple weeks, and if everything goes well I'll switch to Zorin for my personal laptop as well !