Why isn't the read speed across some USB keys constant?

Why isn't the read speed across some USB keys at least approximately
constant?

I have am running Windows XP Professional with Service Pack 1 on
a 2.4GHz Pentium 4 based system with 512MB of main memory.

I have tried both HD Tune 2.52 tab Benchmark
and HD Tach version 3.0.1.0 Long bench (32mb zones)

I get the about the same results for a particular USB Flash
key with both programs.

I noticed that my Kingston Data Traveler Secure DTS/4GB
ran about 2MB/second slower after about 3GB.

I then tested a SanDisk cruzer micro 4.0 GB BH6061GB SDCZ6-4096
and noticed a similar drop, but starting at about 2GB.

I then tested an OCZ Rally 2GB (model number not available) and
it dropped at about 1GB (out of 2GB)

That's strange:
.. some slow at 2GB, some at 1GB, so not an obvious
USB interface problem on the system.
.. some slow 1/2 way through, but some 3/4. Maybe not
a generic flash memory issue, but something to do with
the exact chip layout.

However, I tried my second
SanDisk cruzer micro 4.0 GB BH6061GB SDCZ-4096
It showed a flat read speed over the entire 4GB.

I even switched the SanDisk cruzer micro devices between
two USB jacks on the system and confirmed that the device
itself had the problem, not the slot.

None of the devices are brand new, but I have only used them
for a few benchmarks and copy operations of either a few
large files or tree copy operations. The trees had about
10000 to 20000 files and totaled about 1/2 of the USB
key size in each case.

I didn't do any life tests, like writing the same
block multiple times.

I assume that I am seeing a failure mode, but how can I
isolate it so that I can get warranty replacement units
from the vendors?

There was an article on ExtremeTech's website discussing which USB keys
will work with VISTA's ReadyBoost feature and it explains why not all
USB thumb drives are create equal, check it out:

http://www.extremetech.com/article2/...2017818,00.asp

Key phrase from the article is:

"Another issue is the way flash memory is laid out. Some larger capacity
devices that can be had fairly cheaply get their high transfer rates by
using a small amount of very fast flash memory, with the bulk being
slower memory. In effect, the fast flash is acting as a buffer for the
slower flash."

--
Nik Simpson