Hi Biraj,
I have mounted the USB multiple times in latest Junos versions.
When you plugged the USB, did you get any information in the SRX like below?
root@% umass1: TOSHIBA TransMemory, rev 2.00/1.00, addr 3
da2 at umass-sim1 bus 1 target 0 lun 0
da2: <TOSHIBA TransMemory 5.00> Removable Direct Access SCSI-0 device
da2: 40.000MB/s transfers
da2: 983MB (2013184 512 byte sectors: 64H 32S/T 983C)
If not, I would suggest you to format the USB once again in FAT32 and try tge operation again.