Your mount command is wrong. You should use
mount -t vfat -o sync,users,umask=000 /dev/sda1 /mnt/usbhd
The umask will let any user write to the drive. sync will make sure the data is writen to the medium instead writing to memory. users lets all users un-mount the medium. vfat is fat32.
If you want LINUX to automatically mount your USB medium you have to use hotplug and tweak the script. hotplug is just a bash script to detect USB devices and mount them to your wishes. I suggest you do not add an entry fstab for USB devices because LINUX names each USB device and it will not be the same every time you insert a USB device. Sometimes you may put a USB hard drive first and it will work how you want it when you add the pen drive.
If you want to put an entry in your fstab and forget how it looks in the /mnt directory.
/dev/sda1 /mnt/usbhd users,noauto,sync,umask=000 0 0
Then all you need to do when you insert the USB pen drive, type mount /mnt/usbhd to mount it and umount /mnt/usbhd to un-mount it.