I recently brought a USB flash memory drive, I mounted it on my Redhat 8.0 machine;
# mount /dev/sda1 /mnt/usbhd (as root ofcourse)
and wrote a file to it to make sure it was working, I then took it and installed it on a Windows 98SE machine to make sure I could read the file, I then wrote a file to it on the Windows machine to make sure I could read files I wrote on Windows on Linux.
I plugged the drive into my Linux machine and mounted it, that time and ever since then it has mounted as read only, yet other times I've mounted it on the Windows machine since then it is still read/write.
I tried
# chmod 777 /mnt/usbhd (as root ofcourse)
But it tells me I don't have permission, exact message: (requested: 0777, actual: 0755): Operation not permitted

Can anyone help me out?

Add this to your /etc/fstab:
/dev/sda1 /mnt/usbhd vfat uid=500,gid=500,umask=000,exec,dev,suid,rw 1 0

I am no expert on this. There might be a better (ie secrure) way to do it. But his should get you what you need.