iMac 27“ Mid 2011 - Can‘t reinstall macOS I‘m trying to reinstall macOS Sierra on my Mid 2011 iMac. I have a single 1 TB HDD and single partition called „OS X Base System“ that takes up 999.35 GB‘s. When I try to partition the Base Volume I get the following error 

This volume has 1.56 GB used space. You can not remove the first volume on the disk. This volume can not be split because the resulting
volumes would be too small.


Also when just try to reinstall the OS the disk is greyed out and when hovering on it, it says 

This disk is locked.

Any help would be appreciated.
It sounds like you've somehow installed a MacOS installer disk onto your internal drive. Then you're booting up to that, and trying to install the OS over itself, or trying to partition the volume of the running system.

Boot to Recovery (Command R at restart) and wipe the disk from there. Then install the OS.