I'm not sure what the external drive has to do with anything in your question, but I have both a US and a Canadian iTunes account so hopefully I can help you with your question. I have two because I used to live in the US and now live in Canada.
On the whole, having two accounts is a pain and I don't recommend it unless you REALLY need some media/apps/etc that are only sold in one of the two countries. You have to switch stores in order to buy from this one or that one depending on where its available or where you have the most "credit."
Maintaining the "other" account is simple enough (for me) -- I travel between the two countries regularly, so I remember to pick up US gift cards to keep the US account funded, while my Canadian one is tied to my credit card here.
Except for a few songs I will likely re-buy at some point, I've chosen to keep my music purchases all on the US account just to avoid excessive amounts of switching stores. Since I've authorized both "accounts" to play on this computer, I can play my music on it that was bought from either account, but syncing won't play songs purchased on the Canadian account (since my iPhone's music is "synced" to the US account/Apple ID). You see what a PITA this can get to be already, and I haven't even gotten to apps yet (you have to change stores every time you want to upgrade apps bought on the "other account," though they both sets of apps can run fine on the device).
As for your question about transferring your Canadian-purchased music to your US account, the answer to that is "absolutely not." The only legal way around this I know of is if you're moving to the States and want to play/sync/etc your Canadian-purchased music is a US-based iTunes Match subscription . For one $25/year (which you could cancel after the year), you would get downloadable copies of all the music you bought in Canada or anywhere else (just music; does not apply to other media or apps), now tied to your US account (so you can delete the Canadian purchases). Even if you do not renew the iTunes Match subscription, those songs are yours to keep. Another method would be to simply re-buy the songs from the US account, if available.
Apple flatly does not let users who have changed countries merge accounts (which I wish they would do but I understand the reasons behind that, ie not spite on Apple's part, but due to their agreements with the country's various copyright regulators).
And as you've noticed, gift cards are country-specific. One thing I *do* like about having a US-based iTunes account is iTunes Radio -- which I assume is coming soon to Canada -- it's fantastic. I just today set up a new "ambient" music station centered around the work of Harold Budd, John Foxx (well his ambient stuff), Brian Eno (well his ambient stuff) and it is playing me a number of related artists I am enjoying very much also!
