While you can certainly do what you've suggested...you may want to rethink it.
Personally...this is what I would recommend...
While you can't just use the same drive, attached to your current motherboard, on a new motherboard...unless, of course, that new motherboard is EXACTLY the same motherboard (as the old one) configured the same...same jumper settings...same BIOS, etc., etc...highly unlikely, though...
Your best bet would be to use a new hard drive installed with the new motherboard...partitioned...formatted.. .and setup with the OS of your choosing (as long as it's supported) and jumpered as "Master"...the old drive I'd "Slave". I'd then copy anything I needed from the old drive to the new. That's just me...and I usually stick to my guns...but you may hear different from other users within this forum.
I, too have an external drive, but do not use it as a boot device. I use it as a backup device...files, programs, folders, etc.
My posts tend to get a little long and winded so please bear with me...
Since it is your intention to replace the motherboard...you're better off with the above recommendation...however...you may want to wait to see what others may say...