Try reading these:
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>> btw, ATA/66 is fine as the drive cannot go above 40MB/s
True, both ATA/66 and ATA/100 cannot SUSTAIN above 37MB/sec. However, ATA/100 can get bursts of over 100MB/sec, while ATA/66 cannot attain bursts over ~75MB/sec (usually).
You have some options if...