if it is a WD passport and alike I'd venture to say no, but I could be wrong.
though I have used internal HD as external HD and actually installed Linux OS's to them via the USB port. LiveUSB Linux in one port and the "External" HD in the other USB port and used the laptop CPU / motherboard to install it onto the external HD. in leu of installing the HD into the laptop first then swapping it out with the other original HD.
OS on WD passport?
http://support.wdc.com/KnowledgeBase...r.aspx?ID=9523
using the external HD to run the boot order being Linux, using its grub when it is plugged in. Just update-grub to have the option to pick which to boot up you'd have to have that external HD plug in and using its grub, easy enough, to then have the other OS in its listings to give you the option to pick either one.
it is all the same, just like having one HD split dual boot, or two internal HD's dual booting. you're only changing the means to get the OS to the motherboard then to the CPU to run it.
the benefit being you are actually using two separate HD's one for each OS. it will not mess with your Windows if you do not plug in the external. Windows boot and MBR will be untouched therefore, if you do not plug in your external it will just boot up into Windows without error.
whereas using Windows boot loader to set it up to boot Linux would only give you head aches.
The logic is if you're going to use the external HD then chances are you're going to be using Linux anyways if you plug in your external HD. Therefore, you'll be booting into Linux and not Windows. if you need to get into Windows via Linux it does not even have to be in the grub listings to see it through a file manager to get to it. Or if you want to mount part of it so that part will be in your file manager when you open it up to get into that part of Windows you'd want to use.
that can be done manually on a need by need basis or in fstab.
if you are going to be using Windows then their is no reason to even plugin your external HD, even if you did, because Windows is selfish and Narcissistic it only wants to see itself you would still not be able to use it. Because Linux is not Windows. Their is no real reason to use Windows Boot manager, that is backswords logic.
Think about it.
as far as this goes:
Can I have more then 1 Linux distro on that external drive. It is a blank 2 TB portable drive!!
you can have as many OS's on it as it can fit onto it, but first think it through on how to split it up first before installing anything.
1. what are the sizes of partitions I need for each distro I will be using?
2. Do I need to allow for growth, if yes how much?
3. Do I want a split system for each distro?
/
/home
if yes then do I want to keep things in order of root / and /home
/dev/sda1 swap
/dev/sda2 root / /dev/sda3 /home /dev/sda4 root / /dev/sda5 /home .. etc
or mix it up?
/dev/sda1 swap
Root / partitions:
/dev/sda2 one distro, /dev/sda3 second distro /dev/sda5 third distro /dev/sda6 forth distro
/home partitions:
first distro /dev/sda7 /home second distro /dev/sda8 /home third distro /dev/sda9 /home forth distro /dev/sda10 /home
then go back up to question one and think it through.
@) Can I install more after I get done with this? YES, But I will figure that out when I get there.
these are the questions you need to be asking yourself, and other like questions.
now refer back to what @yancek said about UEFI and/or MBR booting and think about it some more.
