Just putting this here for whoever regarding the File I/O Error: I’ve had it happen two ways. Both ways, the downloading didn’t happen as usual (I last played during the summer, if that helps - the size of the Data folder never goes above about 600 MB for me).
On my C: drive, the File I/O Error occurred during initial setup. I was able to open the client and, after a day or two, got into character select; I could not enter the world without another error (it did not disconnect me like it has in the past when download was incomplete). On a different drive, the File I/O Error did not come up during initial setup and instead is occurring when I log into the account. I enter the credentials, select the server, and then the game crashes and the error box shows up. I don’t play Retail and haven’t for a long time, so this is the only thing WoW related on my computer. I also tried installing on a laptop with the same File I/O Error result on login. I also noticed I had to use the WoW.exe to save my configuration this time around.
If there’s anything on my end I can try, I’ll go for it. Otherwise, I hope that info helps! I’ve seen the other two topics about this, but I’m interested to see if the client is still downloading for other people. It makes me think it could be an issue on my end, but none of the common solutions work (antivirus, admin, etc.).