officerchar :
I deleted it using DiskPart tool in Win10 and it reformated using the same partition unallocated space. Maybe I don't know how to correctly stretch the unallocated space or something. I just want to get back to the default max size of the drive! Is there a recommended third-party software I might need to eliminate the partition (or extend it) which I can't do in Win10 itself?
There shouldn't be any stretch or extend needed.
Have you run the manufacturers diagnostic tool on this drive?
http

/aps2.toshiba-tro.de/kb0/SPD7303VE0000R01.htm