Linux – Ensure USB disk is never sda, even when booting from it

There are many things that seem wrong with your configuration.

1. It doesn't make sense to put USB drives in fstab. Fstab is a mostly static listing of system storage devices and their mount points. You assign a static drive/partition identification (based on controller/device address) to a static mount point. Neither of these are really static for USB drives. In your first example, you'd have problems if you install a second hard-disk to the system: it would become /dev/sdb, moving your current USB drive to /dev/sdc, thus breaking your configuration at boot time.

2. Also, having your drive in fstab and with "auto" option tells the system that the drive is to be mounted at boot time. If for any reason the drive is disconnected at boot time, you'll have your server failing at the boot procedure, which is a really undesirable situation. If the drive is not essential for the system to function, it at least should not have the "auto" option.

3. The "nouser" option is unnecessary, it is the default. The "sync" option is a really bad idea unless you really know what you are doing. Not only it severely impacts performance, it also causes additional wearing of the drive, specially flash media.

4. fs_passno = 3 (sixth column of fstab) causes undefined behavior. Valid values are 0, 1 or 2 only.

5. Using NTFS with a drive intended for backup of a Linux server. Why are you doing that? All Linux NTFS implementations are suboptimal, and inadequate to run any Linux subsystem upon. You should use a native Linux filesystem, like Ext3.

From your kernel log, I could predict that one or more of these may be valid:

1. You have a malpartitioned disk, i.e. the filesystem was formatted with the wrong parameters, bigger than the actual partition; or the partition in the partition table (the logical size) is bigger than the physical size of the drive. This is from the "Buffer I/O errors" you are seeing. You should consider resetting this disk completely, including the partition table, and repartitioning and reformatting it from scratch.

2. You have a bad sector in your disk ("end_request: I/O errors"). You should consider running badblocks over the disk to check.

3. Your USB enclosure is not functioning properly (from "usb resets"). If it is an external hard drive, it should use an independent power supply. The 5V 500mA that is (sometimes) available from the USB port is not enough to keep the mechanics of a hard drive running. It may also be overheating, specially cheap USB flash drives.

4. The NTFS filesystem is corrupted. Fixing it may be possible with ntfsfix, but you should remember, again, that NTFS is not a native Linux filesystem. For best results, you'd need a Windows system to properly check this partition (using Scandisk).

Other suggestions include: stop using NTFS and choose a native Linux filesystem, remove that entry from fstab, manually mount the disk as part of the backup script, umounting it after finished and use UUID or filesystem labels to refer to the disk.

Take a look at the following article at nixCraf, HowTo: Speed Up Linux Software Raid Building And Re-syncing.

It explains the different settings in /proc that can be adjusted to influence the software raid speed. (Not just during building/syncing as the title suggests.)