Ok with NTFS (SP1) it will only support 128GB, hence the reason why yours is saying exactly 128. Sp2 supports up to 2TB. But you said you had installed SP2? But anyway it's not a coincidence that your drive is reportinng exactly 128gb so firstly let me know if u have SP2 or not.

And you can see that it's a Windows issue by going into BIOS and it should correctly report the 300gb.