IO cache makes remaining time inaccurate

When copying files from hard disk to USB disk (writing speed of USB disk is lower than reading speed of my hard disk), Directory Opus uses IO cache/buffer (data read but not written remains in RAM) in first several seconds of copying so Dopus shows copying speed of 50 MB/s. Then the buffer gets full and Dopus starts showing apt. speed (about 20 MB/s). The problem is that it makes remaining time inaccurate because for several seconds it thought that copying speed was much faster and it takes about one minute to calculate real remaining time. Is there any way to fix this?

Dopus 9.1.3.0
Vista Business

Doesn't it average out, after a short time, to a reasonable estimate, unless the file is small in which case it'll be done quickly anyway?

I would like to see how much time will it take shortly after copying has started (in several seconds). Waiting a minute is too long and estimated remaining time changes very much as copying speed "decreases" from 50 to 20 MB/s.
As far as I can remember there was a way to change IO cache on WinXP but as far as I know it doesn't work on Vista.
Any ideas?

I also tried to enable "optimize for fast device dismounting" (in hardware manager, disk properties) instead of "optimize for better performance" (sorry for wrong translation) but no effect.