[quote='e46m',index.php?page=Thread&postID=159146#p ost159146]Would buying an external HD serve as a good solution to this problem, or should I buy another HD for my computer, and get someone install it? Also, would size matter in this case, or is there any particular brand you reccomend (cost effective balanced with quality, please)?[/quote]External HDD would be bad, transfer rates alone would be rubbish.
An internal one would be the way to go, although in your case you might want to try symlinking your WoW directory's.

[url='http://www.dual-boxing.com/forums/index.php?page=Thread&threadID=4854']PowerShell script to create symbolically linked (mklink) copies of WoW[/url] check that out!