Turns out, its only uploads thats the problem.

I can download a 40+ gb file from the usb disk without any problems at all.

@Rofo 

What is the firmware build you are running for the router?

What is also the model of the USB external drive, the size, and file system it is running?

Have you tested with other devices other then the computer running the T9UH? With the computer that has the T9UH, what if you use a Ethernet cable to the router does it still happen?

Typically, I'm now having trouble reproducing the issue I had the other day.

Both uploading and downloading an 8gb file, no problem.

The only thing I did differently the other day was upload hundreds of gigs of data off an external (local) drive, over the network, to the drive attached to the router, so maybe it was just the sheer scale of that copy that made it crash.

For the record, I'm running the latest available firmware for the router model, and the usb disk attached is a seagate 4tb:-

https://www.amazon.co.uk/Seagate-Expansion-Portable-PlayStation-STEA4000400/dp/B017KE8OG0/ref=asc_df_B017KE8OG0/?tag=googshopuk-21&linkCode=df0&hvadid=310843013710&hvpos=1o1&hvnetw=g&hvrand=5448766456551320718&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1006707&hvtargid=pla-315518736178&psc=1&th=1&psc=1

One thing I have noticed.. The drive is capable of over 130mb/s write speed, and higher reads, but the router seems to be capped at 30MB/s write and 56MB/s read, even when the wifi link is 1.3Gb/s (or 162.5MB/s). Doesn't seem to matter if I used the USB 2 or USB 3 socket, and tested with other (faster drives) too, and I hit the same transfer speed caps.

Copying straight from one PC to another directly gives higher speeds. It indicates a limitation with the USB chipset on board the router.

@Rofo 

HQ confirmed that the read and write speeds are slower when utilizing the router. Speed results were similar to what you reported.

Seems the router is still playing up.

Using a remote PC running VLC, I streamed a 1tb file to my TV, from the usb disk attached to the router (See above), and afterwards the router's wifi was still up, but the management console at 192.168.0.1 would not respond, and any devices not already attached with an ip address would not be issued an IP. Rebooting the router fixed it.

Not sure if this is the an issue with the usb interface, so I might try attaching a spare old PC to the router to act as a NAS and see what happens.

@Rofo 

What is the file system you are running for your external hard drive?

@Tony 

NTFS. As its required for large files over 4gb.

What other formats does the router support. I wad thinking of maybe using ext4

@Rofo 

NTFS, FAT32, exFAT, HFS+

@Tony I'm having the exact same issue with my AX50.

Verbatim USB3 500GB drive. Formatted as exFAT.

I found copying large files to it with SMB crashed it quite quickly. Copying to it over FTP seemed more reliable.

One it has crashed, NAT still works (so the kids can still watch Netflix) but you can't join the Wi-Fi network and the Web UI is down. Feels like user space has crashed but the kernel is still up.

I'm running the latest firmware from the website, v1.0.9.

Can I get any logs off it to help?

@thejpster 

If you were to reboot the router and begin copying the files immediately so does it still start to stall?

Do you have other features enabled such as Access Control, Parental Controls, QoS?

There is a feature to lower the USB 3.0 speeds so it does not interfere with the 2.4G performance (Advanced > System Tools > System Parameters). Try disabling that to see if the transfer still gets affected.