[perfect stranger]

Seems to me that this hard drive was an internal drive recognised by the system as Drive “C”.

Yes, it was. Now it's connected as a slave drive and the other partition does not show.

[puppypusher]

If I’m correct I don’t think you will find the other partition unless the Original boot partition is the original drive C:\..

Drive “C” cannot be listed as a slave drive.Since it is the main boot drive.

The only way to find out is to replace the drive the way it was prior to the changes you made.

If you find the sub-partition after the change you will know the data is still there but you won’t be able to access it unless the drives are in their proper order.