Thunderbolt SSD drive speed

Firstly, an ssd is a solid state drive, not a hard drive, it can't be both.

I'm not sure about the effect of Boot camp, but this is from an IT consultant:

Short answer: yes, pretty much

Long answer:

For an internal SSD, there are two potential limiting factors for transfer rate

1. The speed of the drive
2. The transfer interface

At the time of writing, most consumer SSDs run at about the 500MB/s mark, whilst the internal SATA interface is more like 6GB/s. So clearly the bottle-neck there is the drive speed. For the fastest PCIe SSD drives, the speed in my Macbook Pro is more like 2,600MB/s - 5 times faster but still well below the interface’s max speed.

For an external drive there are three potential limiting factors:

1. The speed of the drive
2. The transfer interface system (USB, Thunderbolt)
3. The electronic interface built into the drive (converts SATA to USB, for example)

With the aging USB2.0 running at a theoretical max of 480MB/s and actually running at a fair bit lower, that is the bottleneck and no matter how fast your external drive is, it’s not going to compete with an internal one.

With Thunderbolt 3 at 40GB/s and USB3.1 at 10GB/s, these transfer interfaces are faster than even the high-end PCI-e drives and so the bottleneck is going to be that and/or the drive’s own electronic interface between the drive and the cable. The latter definitely differs between drives. You can buy external SSD drives containing the same SSD, both running on Thunderbolt 3 and yet get different speeds because of this, although the difference is relatively small - like 10–15% usually.

In nutshell, on a modern computer with USB3.1 or Thunderbolt, a standard external SSD drive is likely to be about as fast as an internal SSD drive and if it isn’t then it’s the manufacturer’s cheapo interface fault. You can get external PCIe drives and enclosures but they are much more expensive at the moment. And all of these are faster than internal mechanical drives, which themselves are generally fast enough to perform most duties.

So if you want your Macbook or Windows ultrabook laptop to do video editing, music production or other disk-intensive tasks, using an external SSD, they will perform very well indeed on an external SSD.

Hope that helps.

What is the year/model of your iMac (do not post the serial number)?

On a given interface, macOS and Windows should perform very similarly for raw I/O. When various file systems come into play, they can also impact the read/write throughput (not speed). IO Sizes also matter.

There are many public websites which provide benchmark results.