How odd. It looks like there are two partitions on the USB drive (sda1 & sda2). That may have something to do with it. You should really check via SSH so you can get at the logs via logread &/or dmesg & check status via lsusb & blkid, etc. See the following for a pertinent guide:
I also tried with a second USB drive with a single partition, same result.
Is that really going to show anything different from what I’ve already posted from the system log. Answer, it didn’t while I was ssh’d into the router.
Don’t know about blkid as I didn’t try that, but lsusb isn’t part of the image. Nor is (c)fdisk which I also tried to see what the underlying OS thought about the disks.