CrystalDiskMark is a portable tool which measures read and write performance for your chosen storage device.
The program supports four tests: "Seq Q32T1" (sequential read/ write with multiple threads and queues), "4K Q32T1" (random read/ write with multiple threads and queues), "Seq" (sequential read/ write with a single queue and thread) and "4K" (random read/ write with a single queue and thread).
If you know what you're doing, you can tweak the queue and thread numbers, change the test data, interval times, the test data file size, and more.
But even if you're clueless about most of that, CrystalDiskMark is still very usable. At a minimum you can just choose the drive you'd like to assess (standard hard drives are supported, including network drives, but USB and optical drives are not), hit "All!", and wait for the tests to run.
Once the benchmark is complete, you're presented with read and write speeds in MB/s for each of the four tests (or just the one you selected).
Your results may be saved to disk or copied to the clipboard for reuse elsewhere.
This is the portable version of CrystalDiskMark.