Ed Bott has an excellent post up: Don’t move your Windows user profiles folder to another drive
I’ve always advocated moving your My Documents / Libraries folder to a drive or partition. The method I’ve always suggested is using the Microsoft supported method of using “Move”. See: Move Your Libraries
It seems somewhere there are tweaks being posted that involve moving your Profile folder using the ProfilesDirectory setting. The downside of using this unsupported tweak is “Using this setting to redirect folders to a drive other than the system volume blocks upgrades.” as folks who used this tweak and tried to upgrade to 8.1 just found out. See the article listed above for more info. Well done Ed.
Added to Bad Tweaks.