Linux

SATA III is gigabit, so the max speed is actually 600MB/s.

What filesystem? For example, on my ZFS pool I had to let ZFS use a good chunk of my RAM for it to be able to cache things enough that rsync would max out the throughput.

Rsync doesn't do the files in parallel so at such speeds, the process of open files, read chunks, write chunks, close files, repeat can add up. So you want the kernel to buffer as much of it as possible.

If you look at the disk graphs of both disks, you probably see a read spike, followed by a write spike on the target, instead of a smooth maxed out curve. Then the solution is increasing buffers and caching. Depending on the distro there's a sysctl that may be on by default that limits the size of caches to prevent the "I wrote a 4GB file to my USB stick and now there's 4GB of RAM used for it and it takes hours after finishing the transfer before it's flushed to the stick".

Almost certainly, the bottleneck is one or both of:

1. The platters can’t simply spin at full speed reading a sequential stream of bytes from one and writing it to another - they periodically have to search around to different places stitching the file’s byte stream together from discontinguous chunks or reading or writing metadata. Seek latency of the platter will overshadow any tiny delays incurred because of memory or CPU delays.

2. The algorithm is doing something in a fashion that causes delays (e.g. reading each file individually and waiting until it can sort out if it needs to send anything for that file before starting I/O operations for the next).

Idk if you can do anything about #1 but in similar situations I’ve had good mileage preventing #2 with “tar cj /somewhere | ssh me@host ‘cat | tar xj’” (roughly speaking, you obviously may have to adjust things to make it actually work, and on very fast networks maybe it’s better to skip the -j, but that’s the rough idea).

Edit: Oh, I misread, is this local? I saw rsync and just though it was a network transfer. What kind of speeds are you getting? Does doing “tar c /original | tar x” or something like that work any faster?

Looks like you have your answer, but there are a crazy number of possible issues.

The biggest cause is misreading the performance specs.

A partial list of other options:
Mechanical drives store data in rings. Outer rings have higher speeds than inner due to constant angular velocity.
Seeks cost a lot of throuput on mechanical drives.
Oversubscribed drive cables.
HBA issues.
PCIe data path conflicts
Slow RAM
RAM full or busy
Extra cpy within RAM
NUMA path issues (of drives are connected to different NUMA nodes. Not an issue on desktops.)
CPU too busy
Transfer software doing extra things
File system doing extra.
RAID doing extra.
NIC on a different NUMA node than HBA (can be good or bad).
NIC sharing the data path in a conflicting way.

There are others. Start with checking theoretical performance from data sheets.

Also, details matter, and I don't have enough of them to guess.

You didn't mention if this is a HDD or an SDD. If it's a HDD, you will never even reach SATA 2 speeds, although you should be able to saturate SATA 1. Realistically you might be able to push around 200MB/s on newer HDDs but that's assuming nothing else gets in your way.

Rsync itself may be a bottleneck. Have you compared it to cp command, for instance?

I would put money on rsync being the issue, it's not fast

Are the drives encrypted?

Are any of your resource monitors showing 100% utilization?

What is controlling the SATA drivers? A lot of times the stuff that comes on motherboards isn't the greatest and getting a dedicated storage controller allows you to saturate the drive much more thoroughly. Specially if they have big caches.