This is due to the fact that you installed linux after windows with both drives connected. Your MBR is in sda while your bootloader is in sdb.
You have a few options though
1. You can just clone the first drive using clonezilla-live, then after system is cloned reattach the 500G linux drive & it will boot.
2. you can fix MBR in sda, which will have your linux system unbootable & then repair grub putting it on sdb.
There are a few more options but #1 is what I would do.