Not for those with 2 drives....
Don't bother trying to install this if you've moved your user directories to another drive.
If like me (and many others) you use a SSD as a boot, and have moved the 'USERS' directory to another drive, Micro$oft won't allow you to perform the upgrade.
Only solution appears to be a full wipe and re-install, upgrade to 8.1 and then try to move the directories later.
oh hum......