Teamcity VCS checkout directory (automatically on buildagent) %env.HOMEDRIVE% now inexplicably checking out to wrong drive

Hi,

A buildagent in E:/BuildAgent on Win7(x64) has been running perfectly for weeks.

In the Build Configuration, its' checkout directory settings are:


Since yesterday at 15:00 all testruns have been failing the, single, MSBuild step.
On closer inspection, although I've set the checkout directory custom path to "%env.HOMEDRIVE%/myProject/", the BuildAgent is now actually checking out to C:/myProject.
It should check out to E:/myProject.

Coincidentally, there is also a C: drive on the machine, so the checkout suceeds.
However, the MSBuild step then fails.
The red-highlighted failure error is always:
Cannot start build runner: Unable to find MSBuild path C:\Program Files (x86)\MSBuild\12.0\bin\amd64 defined in build property MSBuildTools12.0_x64_Path
Notice its looking in C: again.

Im starting to think I have a corrupt hard disk.

Any suggestions why the %env.HOMEDRIVE% setting is pointing to C: when the envvars are pointing to E: ?