Steam "disk write error" after reinstalling windows

This turned out to be a permission problem for me. Although there is no mention of permissions on the steam support page for the error at all.

After reinstalling windows, you'll have a user with a different ID. This means your user won't have access to any of your old files. For example I right clicked steamapps, went to properties, security and saw an unknown user with an red question mark icon. So I followed the following answer and things went back to working: Reset File and Folder Permission of External Hard Drive Data to default in windows 7

In summary, run (with cmd prompt with run as administrator): TAKEOWN /F X:\* /R /D Y, followed by ICACLS X:\* /T /Q /C /RESET. (replacing X: with your drive)

Be warned. I could literally reinstall windows faster than these commands take to run. It takes hours!

I'm not sure if completely disabling user access control UAC would fix this and hence be much faster. Running steam as administrator might work too but bugger doing that every time I boot.