Could you copy and paste the log here? Right-click the task, View Log, filter the long to only Errors and above, select the entries, right-click on them and click Copy Selected Entries. Paste the text here.
It's probably an error in the unused drive space erase. If you've got the task still lying around, could you copy and paste the log here for that task? I think the "could not delete Recycle Bin" error is for the full disk task you did.
I don't think so. My guess is that the 123MB left was space the system released after seeing that the disk was getting full. Eraser does try to squeeze every bit of space out of the disk, but sometimes the system outsmarts the algorithm.
I'm guessing that either the Indexing service, or an Anti-virus scan was locking one of the files in there, preventing its proper clean up. Since you don't have the log, then we can only guess.
Are you able to find the folder containing the missing space?
Yes, yes, the little bit of space is normal and expected. Windows creates files which we cannot see in the System Volume Information folder. It's locked up, and unless you do some Windows wizardry, you won't be able to see the contents of the folder. The system keeps persistent volume information there. That usually accounts for the space taken up.
Also, other NTFS file system data files use up the available space, which Windows does not expose to even other programs.