7200 rpm disk is slower than 5400rpm disk?

I have an old PC, motherboard is an MSI MS-6154VA, cpu a 933 Mhz Pentium III, and I just upgraded it to 512 MB of RAM and bought a 40 GB Samsung SP0411N 7200 rpm disk to substitute an older Samsung SV0411N 5400 rpm disk which died about a week ago.

The problem is that after the "upgrade", my HD benchmarks are now lower. With the 5400 rpm disk I used to get the following benchmarks using HD Tach 2.70:

Random Access Time: 15.7 ms
Read Burst Speed: 21.4 MB/sec
Read Speed Maximum: 20.6 MB/sec
Read Speed Minimum: 14.5 MB/sec
Read Speed Average: 17.4 MB/sec
CPU Utilization: 8.2 %

With the 7200 rpm disk I now get:

Random Access Time: 15.6 ms
Read Burst Speed: 16.0 MB/sec
Read Speed Maximum: 15.6 MB/sec
Read Speed Minimum: 13.3 MB/sec
Read Speed Average: 14.9 MB/sec
CPU Utilization: 3.7 %

Even though the motherboard only supports ATA 66, with a single disk this shouldn't be the problem. There should be an improvement with the 7200 rpm disk over the older 5400 rpm one.

During the upgrade, I made the mistake of loading BIOS defaults, which may have caused the problem. BIOS detects the HD as UDMA mode 4, but in the Windows XP Control Panel, the IDE channel properties show it as UDMA mode 3. Can this be the problem?

Another difference is that I now have SP2, which I don't know if this can be causing the problem.

Any suggestions will be appreciated.

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