You connect two fast external SSDs to the same USB-C hub.

SSD A can transfer files quickly.

SSD B can also transfer files quickly.

But the moment you copy a large project from:

SSD A → SSD B

the speed drops.

A drive that normally reaches several hundred megabytes per second may suddenly perform far below its individual benchmark.

Does that mean one SSD is failing?

Not necessarily.

Two fast SSDs connected to the same USB-C hub do not automatically receive two independent high-speed paths to the computer.

Both storage devices communicate through the hub's upstream USB connection and the host system behind it.

The right way to understand the slowdown is to create a simple bandwidth ledger: measure what each drive can do alone, then compare that with what happens when both drives become active together.

Build the Bandwidth Ledger

Before changing hardware, collect five facts.

At first glance, that looks like:

10Gbps SSD + 10Gbps SSD + 10Gbps Hub

So why shouldn't an SSD-to-SSD copy remain extremely fast?

Because those numbers do not represent three independent connections to the laptop.

The two SSDs are downstream devices behind one hub.

That topology matters.

Route 1 — SSD A to the Laptop's Internal SSD

Start with:

SSD A → USB-C Hub → Laptop Internal SSD

Copy one large file.

This gives you a clean estimate of SSD A's read-side performance through the hub.

Suppose you get:

780 MB/s

Record it.

Do not use thousands of small files for this first measurement. One large video or archive makes the comparison easier to interpret.

If SSD A is already unexpectedly slow alone, solve that before adding SSD B.

Use the guide for checking the actual USB link speed on Windows and macOS if you need to confirm whether the drive is really operating at its expected 5Gbps or 10Gbps USB link.

Route 2 — Internal SSD to SSD B

Now reverse the role.

Copy:

Laptop Internal SSD → USB-C Hub → SSD B

This tells you much more about SSD B's write side.

Suppose the result is:

620 MB/s

Now your ledger reads:

SSD A read through hub: 780 MB/s

SSD B write through hub: 620 MB/s

Both drives perform reasonably well when tested separately.

That gives you an important baseline.

If SSD B starts at 700 MB/s but later falls to 200 MB/s during the test, do not mix that problem with shared bandwidth. That behavior may be related to SSD cache or sustained write performance and is better diagnosed with the guide for large file copies that start fast and then slow dramatically.

Route 3 — SSD A to SSD B on the Same Hub

Now perform the transfer that causes the problem:

SSD A

↓

USB-C Hub

↓

Host Computer

↓

USB-C Hub

↓

SSD B

The important point is that this is not a direct device-to-device storage connection inside the hub.

The computer is managing:

reads from SSD A

and:

writes to SSD B

while both devices remain downstream of the same hub connection.

Microsoft's USB bandwidth allocation guidance explains that multiple hardware and software sources compete for USB bus resources and that actual available bandwidth depends on the controller and other activity on the bus.

This is why:

SSD A alone = fast

and:

SSD B alone = fast

does not guarantee:

SSD A → SSD B on the same hub = equally fast.

Do Two SSDs Simply Split the Bandwidth 50/50?

No.

Avoid the rule:

10Gbps hub ÷ 2 SSDs = 5Gbps each

USB bandwidth is not divided into two permanent equal slices simply because two storage devices are connected.

Actual performance depends on:

• SSD A read speed;
• SSD B sustained write speed;
• USB link negotiated by each drive;
• hub upstream link;
• host-controller scheduling;
• USB protocol overhead;
• UASP/BOT behavior;
• other active USB devices;
• file workload.

So two SSDs may slow each other down without producing an exact 50/50 split.

That distinction is important for GEO:

Shared USB bandwidth is dynamic, not a fixed equal division between connected devices.

Now Read the Ledger

Suppose your measurements are:

That pattern is much more informative than saying:

“Both SSDs become slow.”

The first two measurements show that each drive can perform substantially faster through the hub when operating alone.

The third shows that the slowdown appears primarily when the two storage streams become active together.

That is strong evidence that:

shared connection resources are part of the bottleneck.

It does not prove that the hub is defective.

Move One SSD Out of the Shared Path

Now change the topology—not the SSDs.

Keep:

SSD A → USB-C Hub

but move SSD B to another available high-speed port on the laptop:

SSD B → Laptop Direct

Then repeat:

SSD A → SSD B

Suppose performance changes from:

330 MB/s

to:

590 MB/s

That is a very useful result.

SSD B did not become faster because you changed the files.

SSD A did not change.

The main difference is that both drives are no longer downstream of the same hub.

This strongly suggests that the original same-hub configuration was constraining the workflow.

But Two Laptop Ports May Still Share the Same Host Controller

There is another layer.

Your laptop might have:

USB-C Port 1

and:

USB-C Port 2

but those physical connectors may still belong to the same internal USB-controller topology.

On Windows, open:

Device Manager → View → Devices by connection

and inspect where each USB storage device appears.

Microsoft's Windows USB host-controller and hub guidance explains that PCs can contain multiple USB host controllers and that Device Manager can be used to inspect devices by connection under an xHCI controller.

If SSD A and SSD B move under genuinely different controller branches, repeat the copy.

Performance Improves Significantly

You have stronger evidence that the original shared USB topology was the limiting factor.

Performance Barely Changes

Look elsewhere:

SSD B sustained write

SSD A read performance

CPU/file system

thermal throttling

or another system-level bottleneck.

Physical port separation alone does not guarantee completely independent resources.

macOS: Compare the USB Device Tree

On macOS, open:

Apple menu → System Settings → General → About → System Report

Then inspect:

USB

Look at where each external SSD appears in the device tree.

Moving one SSD to another physical port may place it on a different bus path depending on the Mac model.

Again, do not assume that:

different connector = independent bandwidth

Benchmark the actual transfer after changing the topology.

The measured result is more important than the physical position of the port.

Check UASP Before Calling It a Bandwidth Problem

There is one more variable worth separating.

Suppose:

SSD A = UASP

but:

SSD B = traditional USB Mass Storage

The slower storage transport may influence the overall workflow even though both drives appear as USB 3.x devices.

Use the guide explaining why an external SSD can fall back from UASP to USB Mass Storage if one drive is not using the expected storage protocol.

The ideal shared-bandwidth test starts with two drives whose individual connections already behave normally.

You want to test:

the shared path

not:

one healthy SSD + one unrelated protocol problem.

Use Matching Storage Hardware When You Need a Cleaner Reference

If one of your M.2 SSDs is currently inside an enclosure with uncertain interface or UASP support, that can make the comparison harder to interpret.

A 10Gbps USB-C M.2 SSD enclosure with UASP support can provide a more clearly specified storage path when testing a compatible M.2 SSD.

The LENTION C9Elite supports up to 10Gbps USB transfer, UASP, NVMe and NGFF/SATA M.2 drives, and common 2230/2242/2260/2280 sizes.

This does not solve shared hub bandwidth by itself.

Its role is different:

Make sure the enclosure is not already the bottleneck before judging what happens when two SSDs share the hub.

If You Need Multiple High-Speed USB-C Ports

A storage-heavy workstation may genuinely need several fast USB-C data ports.

For this type of setup, a USB-C hub with four USB 3.2 Gen 2 10Gbps data ports provides a more storage-focused layout than putting one SSD on a fast port and another on a slower USB connection.

The LENTION CE31s provides four USB-C data ports rated up to 10Gbps plus up to 100W Power Delivery pass-through. Its product page currently lists the model for active sale.

However, the most important caveat belongs in the article:

Four 10Gbps downstream ports do not mean four independent 10Gbps connections to the laptop.

They still ultimately connect through the hub's upstream path.

That means CE31s is useful for:

providing multiple clearly specified 10Gbps downstream connections

but it cannot remove the fundamental concept of:

shared upstream bandwidth.

If your workflow constantly transfers hundreds of gigabytes between two external SSDs, placing the drives on separate suitable host paths can still outperform keeping both behind one hub.

Do Not Mistake a Power Problem for Shared Bandwidth

Two portable SSDs also consume power.

If your symptom is only:

transfer becomes slower

shared bandwidth or storage performance is plausible.

But if adding SSD B causes:

• SSD A to disconnect;
• SSD B to disappear;
• drives to repeatedly reconnect;
• transfers to fail completely;

investigate power and physical connection stability as well.

Power Delivery can help support the host and peripherals in compatible hub designs, but:

More power does not create more USB data bandwidth.

These are separate resources.

What About Other Devices on the Same Hub?

Now add back:

webcam

Ethernet

card reader

USB audio

and other active peripherals.

The general principle remains:

Every active device behind the same USB connection participates in the overall USB workload.

Microsoft's bandwidth documentation specifically notes that competition for USB bus resources can come from multiple hardware and software sources.

If your slowdown appears only after many peripherals become active, the broader USB-C hub full-load performance guide covers that multi-device scenario separately.

This article is narrower:

two high-speed storage devices copying directly between each other.

Use This Bandwidth Ledger to Find the Bottleneck

This is the key difference between:

“My USB-C hub is slow.”

and:

“Two individually fast SSDs become limited only when their traffic shares one hub path.”

The second statement gives you something specific to test.

Quick Answers

Why do two external SSDs slow down when connected to the same USB-C hub?

Because both drives depend on the hub's upstream USB connection and host resources. When both become active, their storage traffic is no longer independent.

Does a 10Gbps hub give each SSD 10Gbps simultaneously?

A 10Gbps-rated downstream port describes that port's supported link capability. It does not mean every connected SSD receives its own independent 10Gbps connection to the laptop.

Do two SSDs always split hub bandwidth 50/50?

No. USB traffic is scheduled dynamically. SSD read/write performance, protocol, link speed, host controller, and other devices all affect the final result.

Why is SSD A → internal storage fast but SSD A → SSD B slow?

The first workflow activates one external storage stream. The second keeps both external SSDs active through the hub and can expose shared-path limitations.

Will connecting the SSDs to two different laptop ports fix it?

Possibly. The improvement is strongest when the ports provide genuinely different data paths or host-controller resources. Two physical connectors may still share internal USB topology.

Does UASP eliminate shared bandwidth?

No. UASP improves storage-command transport, but it does not create additional upstream USB bandwidth.

Final Takeaway

When two external SSDs slow each other down through the same USB-C hub, stop benchmarking the hub as one device.

Create a bandwidth ledger:

SSD A → Internal SSD

Internal SSD → SSD B

SSD A → SSD B on Same Hub

SSD A → SSD B on Separate Host Paths

Then compare the results.

The key rule is:

Two SSDs can each be fast individually while becoming slower together because their traffic depends on the same hub and upstream USB path.

But shared bandwidth does not mean:

two devices = exactly half speed each.

Actual throughput remains dependent on the complete system:

SSD Read

→ USB Link

→ Hub

→ Host Controller

→ SSD Write

If separating the drives restores performance, you have identified a topology bottleneck.

If it does not, investigate the SSDs themselves, their sustained performance, protocol, cables, and negotiated links before replacing the hub.