Dual boot update to Mint 21 from 20.3
 

Have tried to update Linux Mint 20.3 to 21 on a Lenovo laptop dual boot with Windows 10. On boot get unknown file system. Find that hd0,msdos7 contains the file system but attempting to set root and define linux the command linux /boot/vmlinux root =/dev/sda1 returns unknown command linux. What am I missing out?

What is on the screen when you attempt the linux command? What brought you there? Are you trying to boot the upgraded system? Are you trying to boot the installation media to perform an upgrade? What's on the boot screen? Without enough detail nobody can be much help.

If the / filesystem is on partition 7 the set root should point to it rather than sda1. Alzo, not sure if that is a typo but it is linuz with a z not linux. You indicate (hd0,msdos7) which would indicate a Legacy rather than an EFI install. Is that the case? Are both windows and Mint installed in Legacy mode?

If you don't resolve this, it might be best to get and run boot repair using the 2nd option explained on their page and posting the link you get when it finishes. That will give members here more details to help. Link to boot repair below.

https://help.ubuntu.com/community/Boot-Repair

In grub2, linux is a command that loads a kernel. It's also commonly used as an installation kernel name. Another grub2 kernel loader is linuxefi.

Solved
 

I have resolved it by eventually achieving an OEM install. This took 8 hours to find and initiate as you can see booting from internal disk would not go, neither - to my surprise - booting from a usb stick with Mint 20 on it. I managed to get to the boot menu but could not move the cursor to any option let alone pressing ENTER. I had to use the good old ctrl-alt-del path! The problem was caused by installing Mint 20 on a dual boot (Win 10) laptop. The install option did not give the choice of installing beside Win 10 so I trusted that it would detect the dual environment and behave itself. It did not so it screwed up grub and the original Windows loader. As far as the install was concerned there a mess there and it would not boot. I tried grub rescue but that just confirmed that I was in deep !@#$. I thought of the path of live Linux on a stick; somehow formatting the drive but somebody smiled at be and I was finally able to access the OEM sequence and chose the format option followed by install. I no longer have dual boot but Win 10 is a pain and was actually happy to see it gone.
Thanks for all your input but being a new poster I was not able to clearly describe my situation.
Graham

I was actually referring to the /boot/vmlinux part. The Ubuntu derivatives like Mint that I've seen use vmlinuz or the efi extension. Doubt that was the problem as the info in the last post by the OP indicates no option to install alongside windows. Likely because it was not detected and mounted because it was in the default state (hibernated) but that's just a guess.