I backed up my wallet.dat file along with my seed on a flashdrive which i never plug on another computer just mine with the Win7 OS.
For the best security, your seed should never touch an internet enabled computer, especially if you are storing it in plain text. Even although you are using a different OS, since you are connecting to the internet with that OS you have no guarantee that it has not been infected with some kind of malware. I would store your seed on paper and not on a flash drive at all, or if you insist on storing it on a flash drive, only ever connect that flash drive to a completely air-gapped machine.
Additionally, malware on Windows 10 (Main OS) could read all storage/partition which also used by Windows 7.
However, it's different story if you use Linux (since windows can't read ext4/btrfs by default) or use full drive encryption on your storage.