External Hard Drive Switches Drive Letters

[MatthewSchenker]

Hello,
Can anyone tell me why my external hard drive keeps switching drive letters? When I go into Dolphin one time, it is listed as "sdb1" and then it is listed as "sdc1" the next time.

This would not be a big deal, except for the fact that I created a bookmark for the drive. When the drive letter changes, and I click on the bookmark, Dolphin gives me this message:
Could not mount device.
The reported error was:
No such medium

The only reason I get this error is because the drive letter changes. The drive works fine, and I can access it perfectly from the "Storage Media" bookmark.

Thanks,
Matt

[dibl]

Re: External Hard Drive Switches Drive Letters

USB device designations are "dynamic" -- every time you plug one in, it gets the "next up" device number/letter.

For hard drives and thumb drives, I recommend booting "by UUID" for these USB devices. Here are the last few lines of my fstab, where you can see toward the bottom a NTFS-formatted USB thumb drive that is mounted by UUID on /media/NTFSTICK.

Note that this approach disables the "hot pluggable" character of such devices. I haven't run across a reliable method to have "hot-pluggable" + "always the same ID" working together.

UUID=f5693f79-5640-4d52-a0cd-2db0e0820feb /media/sdd1 xfs nouser,noatime,nodiratime,auto,rw,dev,exec,suid,lo gbufs=8 0 2
# Entry for /dev/sdb1 :
UUID=b311e153-6e03-4b2c-ac17-ebcac0b37ca3 none swap sw 0 0
# Entry for /dev/sde1 :
UUID=A8FC3435FC33FC5E /media/NTFSTICK ntfs nouser,defaults,atime,exec,force 0 2
/dev/scd0 /media/cdrom0 udf,iso9660 user,atime,noauto,rw,dev,exec,suid 0 0
/dev/fd0 /media/floppy0 auto user,atime,noauto,rw,dev,exec,suid 0 0

[MatthewSchenker]

Re: External Hard Drive Switches Drive Letters

USB device designations are "dynamic" -- every time you plug one in, it gets the "next up" device number/letter.

OK, this explains it. So I just need to access my external drive via the "Storage Media" bookmark.

Just out of curiosity, since my main hard drive is always "sda" why isn't the "next up" device always "sdb"? Why would it sometimes be "sdb" and sometimes "sdc"?

Thanks for the help,
Matt

[dibl]

Re: External Hard Drive Switches Drive Letters

I'm not the expert on the USB bus, but I think since USB devices can be all kinds of things other than storage devices (like webcams and scanners, for example), I don't think it is very smart about assigned drive letters to USB things. I think internally it is just the "next up USB thing".

[Snowhog]

Re: External Hard Drive Switches Drive Letters

USB devices first have to be polled. The USB hub is identified first, then it looks for attached devices to the hub. Unlike internal HDs, the 'order' in which USB devices are polled isn't fixed - it varies. When you have more than one USB device attached to the hub, the order, and therefore the numbering, of each device can change on each boot. That explains why (IMO) your external USB HDs aren't listed in your /etc/fstab file - the /dev/identification isn't fixed. If you have only one external USB HD attached to an existing USB port, it will be given a specific /dev/identificaton and will/can be placed in the /etc/fstab file. This is the case with my USB floppy drive, which is connected to it's own USB port. My two external USB HDs are connected via a hub to another of my USB ports. These two HDs are (and can not) be identified in /etc/fstab due to the changing /dev/identification.

[Qqmike]

Re: External Hard Drive Switches Drive Letters

About what dibl said he did:

Back on 3-23-07, I did some basic experiments with UUIDs on UFDs (USB Flash Drives).
(Copied/edited from my files
UUID, fstab experiments. 3/23/07
I was curious about if the UUIDs stay constant. Result: They do.
I played musical chairs with the following factors:
-- 5 UFDs (USB flash drives)
-- Two front USB slots on my PC case
-- Four Linux OSs (two are Kubuntu 6.10 (2.6.17-10 and -11), Freespire 1.0, and MEPIS 6.5))
-- and re-booting

The UUID for a UFD stayed the same, no matter how I tried to confuse it.
NOTE (at that time): my system was two SATA HDDs sda and sdb; WinXP on sda1; and 4 Linux OSs on sdb3, sdb5, sdb6, and sdb8.
- - - - -

That's what dibl is saying, and what I saw with these experiments. You might try similar. The UUID seems to stay with the device. I'd get the same UUID for a UFD in fstab's of both Kubuntu and Freespire (MEPIS used the sdx notation in its fstab, at the time, so it's fstab wasn't helpful with this).

I ran into this situation putting OSs on UFDs: when root=ram0 didn't work, I used root=UUID=XXXXX (ie, the UFD) which always worked (in the GRUB kernel statement).

[Snowhog]

Re: External Hard Drive Switches Drive Letters

These two HDs are (and can not) be identified in /etc/fstab due to the changing /dev/identification.

The above only being true if I don't use UUID to identify them.

[Qqmike]

Re: External Hard Drive Switches Drive Letters

Snowhog--yes, I see. I misread that, as you detected. The UUIds do seem to [always] work. So it seems.

[dibl]

Re: External Hard Drive Switches Drive Letters

The UUID for a UFD stayed the same, no matter how I tried to confuse it.

This is my experience too, Mike.

It's a little painful to give up "hot plugging", but I can live better with a reliable storage device that has to be plugged in at boot time than I can with "hit or miss" storage devices that may not be accessible when I need them.