Why do I only get 24 MB/s throughput when copying files from a USB hard drive to my hard drive?

"24 MBytes/sec is pretty reasonable for a single-SATA -> single-SATA throughput regardless of interface. Especially if there are lots of small files (versus one big file), fragmentation on either disk, etc.

You can use IOmeter to test the maximum sustained read and write speeds for sequential reads and writes. You have to do a full defrag or work against a cleanly formatted partition to get an accurate test."