[AnotherMuggle]

This seems to answer my question:

"Correcting errors in the Volume Bitmap. Windows found problems with the file system.
Issue:

Correcting errors in the Volume Bitmap.
Windows found problems with the file system.
Run CHKDSK with the /F (fix) option to correct these.

Tis is a known issue and the error messages are not actually means that there is problem in your system. You can safely ignore the error messages. The reason you are seeing these errors appear is because when you run CHKDSK in command prompt, it runs in “Read only” mode, and the state of the computer is changing at the time you run the utility. A "read only" chkdsk on an active NTFS volume will result in false positive errors, this is normal.

Read-only CHKDSK will abort before it completes all three phases if it encounters errors in earlier phases and is prone to falsely reporting errors when in read-only mode. That is, CHKDSK may report that a disk is corrupted even when there is no real corruption present. This can happen if NTFS happens to modify areas of the disk on behalf of some program activity that CHKDSK is examining at the same time.

To verify a volume correctly, the volume must be in a static state, and the only
way to guarantee that state is to lock the volume. CHKDSK only locks the volume
when it runs before entering Windows or in Recovery Console with /F or /R (which
implies "F") is specified."