How to Make grub multiboot menu come up only when booting from specific MBR hard drive

Sorry if this is an amateur question. I had an old PC of mine with a 320GB SATA hard disk (lets call it '**sdb**') which had a working Windows 7 installation. Now, i have a laptop which also has a 320GB SATA hard disk with Windows 10 installed on it (referred to as '**sda**'). I bought a SATA-to-USB adapter in order to use **sdb** and install Mint on it. I have repartitioned it like this:

[100 MB System Reserved Windows 7] [50GB Windows 7] [unallocated 70GB (to-be Linux Mint partition) [177GB Data Partition]

Structure of **sda** is:

[350MB System Reserved Windows 10] [97GB Windows 10] [98GB Local Disk D:] [102GB Local Disk E:]

I haven't yet installed Linux Mint. What i want to do is: 1) leave the internal HDD (sda) untouched 2) install Linux Mint on the 70GB space. 3) Make it so that the GRUB multi boot menu comes up ONLY when i set BIOS to boot from the USB drive 4) (i) **[IMPORTANT]** when the USB HDD is disconnected and laptop is booted, the laptop should go directly to Windows 10 with no interruptions. (ii) But when booted from the USB drive (after selecting it as the boot drive from BIOS), GRUB boot menu should come up to ask whether to boot Mint or Windows 7 5) I dont mind if Windows 10 comes up in the boot menu. boot-info paste: https://paste.debian.net/hidden/26b1d460/ The questions is: What am i supposed to select under 'Select device for bootloader installation' in order to achieve my goal? Or how do i actually go through the Mint installation for this purpose? Notes: Both Hard Drives are MBR and the laptop is HP Elitebook 8470p. Windows 7 exists only for the sake of archival (wont be booted usually). If possible please do not suggest removing the internal HDD and then install Mint. Related questions: https://unix.stackexchange.com/questions/66895/grub-with-two-hard-drives https://unix.stackexchange.com/questions/374514/grub-booting-only-when-the-drive-is-explicitly-selected