Systemd Unmounting Removable Drive – Troubleshooting

The problem is that your /etc/mtab is not a file but a symlink to /proc/mounts. This has advantages but also the disadvantage that user does not work. You already guessed right the reason for that: "the system is confused when remembering who mounted the file system". This information is written to mtab, cannot be written there in your case though. The kernel doesn't care (doesn't even know) about user mounts (this is a userspace feature). Thus this info is not contained in /proc/mounts.

Do this:

cd /etc
cp mtab mtab.file
rm mtab
mv mtab.file mtab

umount as user should work after you have mounted the volume again.

The link /dev/$disk points to the whole of a block device, but, on a partitioned disk without unallocated space, the only part which isn't also represented in /dev/$disk[num] is the first 2kb-4mb or so - $disk's partition table. It's just some information written to the raw device in a format that the firmware and/or OS can read. Different systems interpret it in different ways and for different reasons. I will cover three.

• On BIOS systems this table is written in the MBR master boot record format so the firmware can figure out where to find the bootable executable. It reads the partition table because in order to boot BIOS reads in the first 512 bytes of the partition the table marks with the bootable flag and executes it. Those 512 bytes usually contain a bootloader (like grub or lilo on a lot of linux systems) that then chainloads another executable (such as the linux kernel) located on a partition formatted with a filesystem the loader understands.

• On EFI systems and/or BIOS systems with newer kernels this partition table can be a GPT GUID partition table format. EFI firmware understands the FAT filesystem and so it looks for the partition the table describes with the EFI system partition flag, mounts it as FAT, and attempts to execute the path stored in its Boot0000-{GUID} NVRAM variable. This is essentially the same task that BIOS bootloaders are designed to do, and, so long as the executable you wish to load can be interpreted by the firmware (such as most Linux kernels since v. 3.3), obviates their use. EFI firmware is a little more sophisticated.

After boot, if a partition table is present and the kernel understands it, /dev/${disk}1 is mapped to the 4mb+ offset and ends where the partition table says it does. Partitions really are just arbitrary logical dividers like:

start of disk | partition table | partition 1 | ... and so on | end of disk

Though I suppose it could also be:

s.o.d. | p.t. | --- unallocated raw space --- | partition 1 | ... | e.o.d. 

It all depends on the layout you define in the partition table - which you can do with tools like fdisk for MBR formats or gdisk for GPT formats.

• The firmware needs a partition table for the boot device, but the kernel needs one for any subdivided block device on which you wish it to recognize a filesystem. If a disk is partitioned, without the table the kernel would not locate superblocks in a disk scan. It reads the partition table and maps those offsets to links in /dev/$disk[num]. At the start of each partition it looks for the superblock. It's just a few kb of data (if that) that tells the kernel what type of filesystem it is. A robust filesystem will distribute backups of its superblock throughout its partition. If the partition does not contain a readable superblock which the kernel understands the kernel will not recognize a filesystem there at all.

In any case, the point is you don't really need these tables on any disk that need not ever be interpreted by firmware - like on disks from which you don't boot (which is also the only workable GPT+BIOS case) - and on which you want only a single filesystem. /dev/$disk can be formatted in whole with any filesystem you like. You can mkfs.fat /dev/$disk all day if you want - and probably Windows will anyway as it generally does for device types it marks with the removable flag.

In other words, it is entirely possible to put a filesystem superblock at the head of a disk rather than a partition table, in which case, provided the kernel understands the filesystem, you can:

mount /dev/$disk /path/to/mount/point

But if you want partitions and they are not already there then you need to create them - meaning write a table mapping their locations to the head of the disk - with tools like fdisk or gdisk as mentioned.

All of this together leaves me to suggest that your problem is one in these three:

• your disk has no partition table and no filesystem

  • It was recently wiped, never used, or is otherwise corrupt.

• your disk's partition table is not recognized by your os kernel

  • BIOS and EFI are not the only firmware types. This is especially true in the mobile/embedded realm where an SDHC card could be especially useful, though many such devices use layers of less-sophisticated filesystems that blur the lines between a filesystem and a partition table.

• your disk has no partition table and is formatted with a filesystem not recognized by your os kernel

After rereading your comment above I'm fairly certain it is the latter case. I recommend you get a manual on that tv, try to find out if you can get whatever filesystem it is using loaded as a kernel module in a desktop linux and mount the disk there.