Need Help installing Bootloader

I am trying to install ubuntu alongside Windows. I am installing on a different drive that does not have windows on it. I have partitioned the drive (the one im installing ubuntu on) so half is for ubuntu and half is for extra storage for windows.

I have read everywhere to install the bootloader on /dev/sda but there is a problem with that. My /dev/sda is an external drive I use for backups.

My Windows drive is /dev/sdb and the drive im installing ubuntu on is /dev/sdd. Which one do I install the bootloader on?

This is how my drives appear in the menu:

/dev/sda  Seagate Expansion Desk (2.0 TB)
/dev/sda1
/dev/sda2
/dev/sdb  ATA PNY CS900 120GB (120.0 GB) [This is windows]
/dev/sdb1
/dev/sdb2
/dev/sdb4
/dev/sdc  ATA WDC WD5000AVVS-6 (500.1 GB)
/dev/sdc1
/dev/sdd  ATA WDC WD10EADX-22T (1.0 TB)
/dev/sdd1
/dev/sdd5 [ubuntu install partition]

Thanks for your help!