For a complete benchmark suit in Linux, I would recommend lmbench. You will have the benchmark result for CPU, memory, network, disk etc.

Simple Linux Disk Benchmark

Actually, you can rely on dd for simple disk performance evaluation

Test Write speed

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dd if=/dev/zero of=tempfile bs=1M count=1024

Test Read speed from buffer

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dd if=tempfile of=/dev/null bs=1M count=1024

Teest Real Read speed

Just clear the linux cache, then run dd

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sysctl -w vm/drop_caches=3
dd if=tempfile of=/dev/null bs=1M count=1024

Test on external hard drive

External HD includes external HDD, USB flash drive, removable device etc.

Replace tempfile in the above command by your mount point

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dd if=/dev/zero of=/media/user/MyUSB/tempfile bs=1M count=1024