USB Drive Size, File System Recommendations, and Time Machine on Asus Merlin Routers

[KCi_MK]

While browsing the forums, I noticed some confusion regarding the maximum supported drive size, the best file system to use, and how to set up Time Machine on Asus Merlin routers.

I tested and confirmed the following on the GT-AX11000, GT-AXE16000, and GT-BE98 models:

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Quick Summary:​

1. Maximum Drive Size:
   • You can use drives of any size, including partitions larger than 4TB.
2. Formatting and Partitioning:
   • Do NOT use the Asus (or Merlin) web interface to format or partition your drive.
   • It limits partitions to 2TB and only supports formatting in FAT, NTFS, and HFS.
   • The recommended file system for speed and data consistency on these Linux-based routers is ext4.
3. Why Use ext4:
   • Specifically developed for Linux, ext4 supports volumes up to 1EB and files up to 16TB.
   • Best performance (lower CPU usage) and more resilient against data corruption than NTFS, FAT, or HFS.
   • NTFS and HFS drivers, provided by Tuxera, are not native to Linux and should be avoided for read-write operations unless necessary.
4. Time Machine Support:
   • Time Machine (Under USB Application->Time Machine) (=AFP; Apple Filing Protocol) works properly, even on the latest macOS 15, if your USB device is formatted correctly for the router’s Linux kernel.
   • However, since AFP is deprecated (starting with macOS Sierra 10.12), Apple now recommends using SMB (version 3.x) for Time Machine backups. Future macOS versions may eventually drop AFP support altogether.

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How to Prepare a USB Drive for Asus Router (Using ext4)​

I use Fedora Linux for partitioning and formatting, but any modern Linux distribution will work. The router’s kernel doesn’t support all the latest ext4 features, so it’s best to format the partition directly on the router.

Steps:​

1. Connect to the Router Terminal:
   • SSH into your router.
2. Connect the USB Drive:
   • Plug the device into the router’s USB port.
   • It won’t automatically mount due to ext4 incompatibilities.
3. Check the Partition Table:
   • Run:
     

     fdisk -l

   • Ignore the warning:
     

     fdisk: device has more than 2^32 sectors, can't use all of them

   • Find your partition, usually number 1, named something like /dev/sda1.
4. Format sda1 with ext4:
   • Run:
     

     mke2fs -t ext4 -L DEVNAME /dev/sda1

   • Replace DEVNAME with a recognizable label for your drive.
5. Re-mount the Drive:
   • Unplug and reconnect the USB drive.
   • The router should now automatically mount it.

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​

Optional: Verify and Manually Mount the Drive (Advanced Users)​

1. Monitor System Logs:
   • Keep an eye on the system log during the process to catch any issues.
2. Create Mount-point and Mount the Drive:
   

   mkdir /mnt/DEVNAME
   mount /dev/sda1 /mnt/DEVNAME

3. Unmount and Clean Up:
   • If everything works as expected:

   • umount /dev/sda1
     rm -r /mnt/DEVNAME

   • This prevents potential "ghost" mount points.
4. Re-plug the USB Drive:
   • The router should automatically mount it, and the device will be ready for use.

 

[ColinTaylor]

Also see the Wiki which covers a lot of the same information.

Disk formatting

Third party firmware for Asus routers (newer codebase) - RMerl/asuswrt-merlin.ng

github.com

 

[bennor]

There is what Asus indicates (link below) for; Disk Format, Maximum Partition Size, Maximum Partition Number and File Size, and then there is what people will do in the real world (as evidenced in many past discussions on USB drives, capacities and partition type).
ASUS Router Plug-n-Share Disks Compatibility List

 

[bbunge]

Even so, a router does not make a good NAS. The larger the drive the greater chance for data loss. USB 3 devices can cause issues with 2.4 GHz WIFI. Small thumb drives or small SSD's, yes. Large spinning rust, no!

 

[Tech9]

How to Prepare a USB Drive for Asus Router

There is much shorter method saving future trouble - don't plug the USB drive into this Asus router. When you test it only it will be working, when you actually need the backup data you may find it corrupted. This is not written in the Wiki.

 

[KCi_MK]

A bit more info on 2.4GHz interference with USB 3: https://www.usb.org/sites/default/files/327216.pdf

Agree, spinning HDD's in this configuration is asking for trouble.
SSDs, however, are a different story; they’re becoming larger, more affordable, and often come with high-quality shielded cables to minimize issues.
If substantial storage is needed, investing in a proper NAS is the best approach. That said, what qualifies as "large" storage today is quite different from just a few years ago.

 

[KCi_MK]

MacOS users should generally not worry about Time Machine backup being corrupt.

MacOS performs consistency checks on Time Machine sparsebundle files, as follows:

1. During Backups:
   Time Machine verifies the integrity of individual files as they are written, ensuring that data is correctly copied to the backup. However, it doesn’t check the full integrity of the entire sparsebundle after every backup session.
2. Periodic Verification:
   macOS runs full consistency checks periodicallyor when certain conditions are met, such as:
   • The backup disk hasn’t been used for a while.
   • Time Machine detects potential issues (e.g., sudden disconnection, failed backup attempts).
   • After a forced shutdown or power loss.
3. Manual Verification:
   You can trigger a full consistency check manually using Disk Utility or the Terminal command:
   
   

   sudo tmutil verifychecksums /path/to/your/backup.sparsebundle

   This will verify the integrity of the backup without making changes.
4. Automatic Repair Attempts:
   If macOS detects corruption during a backup, Time Machine will usually attempt to repair the sparsebundle. If the corruption is severe, you might be prompted to start a new backup.

Backups should be stored on at least three different media, with at least one off-site. This approach ensures data redundancy, protects against hardware failures, and safeguards the data from physical threats such as theft, fire or other disasters. There is nothing wrong, if one of those media is an SSD connected to Asus router. It is one of three anyway.
I would not rely on it as the sole backup medium. If it is the only backup option, connecting the USB drive directly to the local workstation hub is much faster and more convenient.
Additionally, sparsebundle files are encrypted, providing strong protection against third-party access—assuming a robust password is used.