not only in 4.3, i have faced this problem in 4.4 omega and arhd also, with original or agni kernel vice versa.
When i insert usb drive with otg, i see the " mass storage mounted" etc. in the screen but have to wait 3-7 minutes in order to the files in otg be seen in a file explorer (samsung or es file manager). If the flash drive is empty i havent got this problem.
However in twrp recovery there is not any wait duration and usb can be used after mounting.
I think phone scans the files in the usb storage after each mount. I placed .nomedia file in to the root folder of usb but nothing changed. Is there any way to walk around this problem, or is it related with android media scan?
edit: ok, after deleting portableapps folder from the drive solution fixed. Android scans all files and having 40k files slowed the scan. If i found a solution to disable scan i will write here.