I have buffalo hd-hs500u2 and i cant transfer big file,,

Both devices must have USB 3.0 capability to enjoy the high transfer speed that 3.0 has to offer. Check the other device (not the Buffalo) as see if it has USB 3.0 support.
First, make sure the firmware for the external and the source device are up to date. Next, try swapping the cable.
If that doesn't work, check the source and target hard drives to symptoms of failure. The transfer speed will be slow if one of the drives is failing. The manufacturer of the drive should have free diagnostic tools on their website. If not try WD Lifeguard Diagnostics or Seagate Seatools. If you are running Linux, go with smartmontools.
If the drives are healthy and if 3.0 is supported on both devices check the transfer speed with different files. Transferring one large file is faster than transferring a lot of small files.
If all else fails call Buffalo and request a replacement.

The file is too large meaning that the size of that SINGLE file is too large. Your hard disk is probably formatted to a FAT32 file system, this results in a maximum of 4gb PER FILE maximum transfer. What you could do is archive it into a few parts using winrar and then moving it onto the disk, and when you need the file just extract the archive. Your problem isn't the total free space, it's the size of that file chunk.

go to system performance and settings and try to format from there

You can download the CD image of the files you need here. You just need to put in your model number:
http://www.buffalotech.com/support/downloads/

papsikle

psp

Is it "slow" only on your computer, or also on some other computer?

If it _consistently_ is slow, then it probably has at least a one-year warranty. So, return it to where you bought it, and get it repaired/replaced.

Try plugging it into a different USB port.

my Buffalo external hard disc can be detected by laptop, but my access is denied. When i click on the hard disc icon, it popped out a message box prompting me to reformat my hard disc. Is there any other way to retrieve my files before reformat or
is there any other way to recover my files after reformat?
Is there any other way to troubleshoot my hard disc other than reformatting it?
Please help me, I am desperate to recover all my files because they are too important/precious to lose it.

plz try to format the drive before transfer....