我正在关注这个教程。但我卡在了第 9 步，不知何故我无法安装我的 USB。
每次我尝试安装 USB 时都会收到以下错误：
root@OpenWrt:~# mount /dev/sda2 /mnt/sda2
mount: mounting /dev/sda2 on /mnt/sda2 failed: Invalid argument
USB 似乎正常并已连接，但不知何故我无法安装 sda2 (ext4) 分区。
分区似乎也没有问题：
root@OpenWrt:~# blkid
/dev/mtdblock2: TYPE="squashfs"
/dev/sda1: UUID="e39964e8-1b51-4b1f-b034-0147fa394eea" TYPE="swap"
/dev/sda2: UUID="157cfc0d-f33d-4103-950d-6ae01baa7177" TYPE="ext4"
这是我的dmesg输出：
root@OpenWrt:~# dmesg | grep sda
[ 9.360000] sd 0:0:0:0: [sda] 7987200 512-byte logical blocks: (4.08 GB/3.80 GiB)
[ 9.370000] sd 0:0:0:0: [sda] Write Protect is off
[ 9.370000] sd 0:0:0:0: [sda] Mode Sense: 23 00 00 00
[ 9.370000] sd 0:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ 9.400000] sda: sda1 sda2
[ 9.410000] sd 0:0:0:0: [sda] Attached SCSI removable disk
[ 41.850000] EXT4-fs (sda2): couldn't mount as ext3 due to feature incompatibilities
[ 41.870000] EXT4-fs (sda2): couldn't mount as ext2 due to feature incompatibilities
[ 41.880000] EXT4-fs (sda2): couldn't mount RDWR because of unsupported optional features (400)
[ 270.660000] EXT4-fs (sda2): couldn't mount as ext3 due to feature incompatibilities
[ 270.670000] EXT4-fs (sda2): couldn't mount as ext2 due to feature incompatibilities
[ 270.670000] EXT4-fs (sda2): couldn't mount RDWR because of unsupported optional features (400)
[ 585.040000] EXT4-fs (sda2): couldn't mount as ext3 due to feature incompatibilities
[ 585.050000] EXT4-fs (sda2): couldn't mount as ext2 due to feature incompatibilities
[ 585.060000] EXT4-fs (sda2): couldn't mount RDWR because of unsupported optional features (400)
[ 586.540000] EXT4-fs (sda2): couldn't mount as ext3 due to feature incompatibilities
[ 586.550000] EXT4-fs (sda2): couldn't mount as ext2 due to feature incompatibilities
[ 586.560000] EXT4-fs (sda2): couldn't mount RDWR because of unsupported optional features (400)
[ 651.570000] EXT4-fs (sda2): couldn't mount as ext3 due to feature incompatibilities
[ 651.580000] EXT4-fs (sda2): couldn't mount as ext2 due to feature incompatibilities
[ 651.590000] EXT4-fs (sda2): couldn't mount RDWR because of unsupported optional features (400)
这是我的fdisk -l输出：
WARNING: GPT (GUID Partition Table) detected on '/dev/sda'! The util fdisk doesn't support GPT. Use GNU Parted.
Disk /dev/sda: 4089 MB, 4089446400 bytes
255 heads, 63 sectors/track, 497 cylinders, total 7987200 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000
Device Boot Start End Blocks Id System
/dev/sda1 1 7987199 3993599+ ee GPT
错误消息来自以下行<Linux kernel source code>/fs/ext4/super.c：
if (ext4_has_unknown_ext4_ro_compat_features(sb)) {
ext4_msg(sb, KERN_ERR, "couldn't mount RDWR because of "
"unsupported optional features (%x)",
(le32_to_cpu(EXT4_SB(sb)->s_es->s_feature_ro_compat) &
~EXT4_FEATURE_RO_COMPAT_SUPP));
错误消息中的数字的占位符是%x，因此实际错误消息中的数字 400 是十六进制的 0x400。如果特征由EXT4_FEATURE_RO_COMPAT_常量识别，则它是一种。
这些常量的定义可以在<Linux kernel source code>/fs/ext4/ext4.h：
#define EXT4_FEATURE_RO_COMPAT_METADATA_CSUM 0x0400
dir_nlinksourcejedi提到的特征是0x20，而不是0x400 ：
#define EXT4_FEATURE_RO_COMPAT_DIR_NLINK 0x0020
因此，MR3020 无法以读/写模式挂载文件系统，因为其操作系统无法处理 ext4 元数据校验和。因此，您需要使用用于创建 USB 记忆棒的 Linux 系统关闭该功能。将 USB 记忆棒移回该系统（不要挂载 ext4 文件系统）并运行：
tune2fs -O^metadata_csum /dev/sdX2
（将 X 替换为 Linux 系统中 USB 记忆棒的实际标识符。）
成功运行此命令后，该metadata_csum功能将在 USB 记忆棒的 ext4 文件系统中被禁用，并且 MR3020 现在应该能够使用它。