fix linux boot, multiple hard drives
 

I have windows 10 installed on my primary hard drive, which is a SSD with limited storage.

I then installed linux mint by unplugging the SSD, and letting the installer just do its thing. On the remaining hard drive, there was already a windows 7 installation. Linux is now installed alongside with Windows 7.

Then, using easyBCD, I added the linux installation as a boot option, along with the existing windows 10 and 7.

But the boot doesn't work. when I choose linux, it just restarts the PC and brings me back to the multiple boot options.

I assume this is because grub is installed on a different hard drive.
how to fix?

just to clarify:
I did boot successfully to linux, before I reconnected the SSD.

the problem is only when I try to do it from the multiple boot menu that I set up with EasyBCD: I can boot to windows, but not to linux.

I know that I could boot to linux by going into BIOS and setting the second hard drive as primary, but I prefer to have linux as a regular boot option, alongside with windows.

Did you install windows 10 as UEFI? Did you then install Mint using UEFI? If not, that could be part of the problem. You could try going to the site below and downloading the boot repair iso and burning it as a bootable image to a CD. After booting it, there should be an option to "Create Bootinfo summary" and if you select that, it should output a text file with detailed information on your drives/partitions and boot files.

http://sourceforge.net/projects/boot-repair-cd/files/

I'm not sure I'm clear but if I understand your post, if you set the secondary drive with Mint to first boot priority, you can boot it?
If you try to boot with EasyBCD from the primary drive, it fails?

Hi,
I did not install anything with uefi. I have an old-style bios mobo. everything is without uefi.

if I go into the bios settings and choose the HDD, then it will boot to linux.
if I leave it alone and it boots from the SSD, then I get to the multiple boot menu of EasyBCD. If I choose linux from the list, it will not boot.

fixed!
I went back into easybcd, I chose grub2 instead of grub legacy. And probably more important, instead of letting it find the boot partition automatically, I selected it from the list.
now it works like a charm.

Multiple MBRs
 

What probably happened:

You ran your dual boot install with your SSD removed, so grub wrote its info to /boot/grub and to the mbr (master boot record - 1st small part of disk where the bootstrap loader gets saved) on the secondary drive.

When you connected your SSD again, it became recognized as the 1st drive and was used to bootstrap the computer, but you had not modified it at all because it was disconnected.

Now when you ran grub again, it found your second drive and the new OS partition on it and added it to the boot menu and wrote it to /boot/grub on the SSD.

Upon rebooting, your system used the updated menu from your SSD, so the new OS was found.

Most likely it didn't have anything to do with the version of grub you used, just about where grub wrote the information and where your system subsequently looked for it when rebooted.