I’ve posted my problem on the Arch forums, but the topic was closed, oops :p
I’ve observed this issue since kernel 4.5.1 I believe. I’m now using the lts kernel (4.4.9) and everything is fine.
My setup: desktop computer with
-sda: internal SATA SSD (Arch system and other system partitions)
-sdb: internal SATA HDD (windows data partition)
-sdc: internal SATA HDD (NTFS shared data partition and EXT4 extra linux partition for Steam games)
-sdl: external USB3 HDD (NTFS data partition)
My fstab configuration works fine, ie: system, sdc1, sdc2 and sdl1 are mounted on startup.
There are 2 problems:
1: The drives are not seen as devices, both those that are mounted at startup and those that are not, except sdc2 (EXT4 partition) which is seen. They are not in the device list in nemo, nautilus and the removable drive applet, both in Gnome 3 and Cinnamon if that matters.
2: Automount of external drives does not work. When I plug in a USB drive or SD card (in USB 2 or 3 port), it is not automounted and it does not appear in the device list. The exception is my Samsung S3 phone that is automounted and appears in the device list.
I can see everything in the gnome-disk-utility and mount drives through there (they appear in the device list once I mount them). Running sudo fdisk -l lists everything.
The lsblk -f output is exactly the same with both kernels, except that when I plug in an USB drive, it automounts with the lts kernel and is only listed (not mounted) with the mainline kernel.
Another thing: with kernel 4.5.4 (and since 4.5.1), I see this message not long after going through GRUB (right after “starting version 229”):
“No Caching mode page present. Assuming drive cache: write through” about sdl (my external USB3 HDD)
I don’t see this message with the lts kernel (4.4.9).
By the way, this is my “device” list in nemo with kernel 4.5.4
And with kernel 4.4.9
Strangely, the only device (sdc2 /mnt/HDD) that shows up with kernel 4.5.4 is the only device that does not show up with kernel 4.4.9
I have another computer that does not have this problem (a laptop).