|"I have a pen drive which is not open properly..."|
You need to explain what you meant when you made that statement better.
Some models of flash drives have a tiny switch on them that must be moved to a certain position in order to be able to write data on the flash drive, or if you want to to format the drive.
If your model doesn't have that, it has software write protection, and you need to provide the make and model of the flash (pen) drive.
"...when i plug in the USB pen drive then it shows for format n in the Capacity's tab shows "1.99TB", ..."
I know of NO maker that makes a 2 tb flash drive, and if they DID a REAL one would be really expensive and would NOT be small. There have been rumors of them coming out for about a year, but I know of NO maker yet that has come out with one
However, I have found mentions of FAKE 2 tb flash drives being sold on the web.
If it's physically really small, or if it was relatively cheap to buy, it's a FAKE !
"File system's tab shows "exFAT(Default)", & in Allocation unit size's tab shows "512kilobytes".
A REAL 2 tb flash drive wouild likely NOT be using 512 kb allocation units.