Removing second hard drive after installtion

Hello.
First off, this post is about an linux mint installation.

I have two hard drives on my computer, sda and sdb. sda is old and has windows XP and linux mint 17. sdb is brand new and I installed linux mint 18 on it. GRUB is installed on sdb.
I wanted to remove the old hard drive after installing LM 18 on the new HD. However, once I remove the old hard drive, there is a significant increase in boot time... it takes about 3 - 4 minutes. While the old drive is plugged in, boot takes less than a minute.
Why is unplugging the old hard drive producing longer boot times? there is nothing else wrong with the boot procedure.. no errors are shown.

(at one point, in order to fix this issue, I unplugged the old hard drive and then ran update-grub.. the update was successful but it didn't solve this issue. It also reduced the resolution of the virtual TTYs but i fixed that by editing /etc/default/grub)

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