After Debian installation, how to add Grub Entries for new/other bootable disks/installations?

It depends on which boot-loader was installed. If its a standard Debian install it should be GRUB2.

Boot the computer with all disks containing bootable installations attached and powered.

you need to open Root Terminal application to open a terminal as root, then enter these commands:

apt-get update
apt-get install os-prober

if os-prober package is already installed, apt will let you know, without doing any changes to the system.

Then edit /etc/default/grub and make sure you have a line like

GRUB_DISABLE_OS_PROBER=false

you can edit the file using a GUI text editor like Gedit, or a CLI Editor, such as Vim or Nano.

Using Gedit:

gksu gedit /etc/default/grub

You need to close gedit to be able to use the terminal again

Using Nano

nano /etc/default/grub

I don't recommend using vim if you're a beginner, it takes some time to get used to it's operation modes and interface.

Once you're done with editing the file, if necessary, enter this command

update-grub

Note: You can skip the file editing process on your first try, but if that doesn't work you'll need to do it, then retry update-grub command.

You could:

It should detect Windows on the first Disk, and add an entry to the boot-loader choices.

And you could then optionally install grub also to the other disk boot sector by running grub-install /dev/sdb

sdb is what I assume to be the device node for the second attached disk, else try sda

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