I wonder what decide usb as sda or adb.
Linux shows the usb as sda at first.
But it shows the usb as sdb at second time after I reinsert the same usb.

Please help me, I can't understand what happen to this.

Hey there!

Welcome to the forum, and, wow, you set the tone with an interesitng one.
So, the first disk detected is likely the one the system booted from.
Now...

This, you dont decide, the system does that for you...

Possibly because that's the boot device...

Obviously...that's a disk that came in later on...

...the first step to wisdom is understanding...that you dont understand everything...
(same with me, I still learn everyday, such wonder is Life...)

Thor

This is normal behaviour for modern Linux systems. The order if which device names are assigned is not guaranteed.

Have a look at this: Persistent block device naming

I think I didn't explain it enough.

1. I inserted usb A, and Linux detected it as sda.
2. I extract usb A.
3. I did "umount -l" after extract the usb.
Because "umount" did not work after the extration like below.
4. And I reinserted the same usb A.
But, Linux detect it as sdb. Even the system has only one usb.

I think something did not cleaned perfectly.
Because, Linux is writing on the usb when I extract it from the system.

This is a really bad idea! Never unplug a device that is being used. You need to umnount it before removing it, not doing so will have strange possible side effects:

- data might become corrupted,
- the system still thinks the device is still present (which explains it showing up as sdb).

Again: _Never_ unplug a device that is still mounted. Unmount it and then remove it.

Thank you druuna.
Is there any way to make the linux thinks the usb removed?

Bit of an ambiguous question...

- Are you talking about removing the entry after wrongfully removing the device (sda in your case)? If so:
There's no real need to do this manually. The system will pick up on it eventually and do some housecleaning itself.

- Or are you asking how to unmount a device? If that is the case:
If you use a GUI the right click on the device an umount option should be present, if you use the terminal then use the following command: The X can be a, b, c etc and the Y is a number, which is shown by the df -h command.

An example of the umount command:

Thank you Druuna.
I was talking about removing the entry after wrongfully removing the device.
Because my system detect the same usb as sdb after abnormal removing.
I thought the reason is that the linux thinks the usb still present.

But judging by the account of you,
If I want to detect it as sda again, the only thing I can do is just waiting for cleaning.
Am I right? It's too sad.

If you wrongfully remove the device then Linux _does_ think it is still present.

Yep (or reboot )

Well, you are to blame. As stated before: Always unmount a device before removing it.

OK. Thank you druuna.

You're welcome

BTW: Can you put up the [SOLVED] tag (upper right corner or Thread Tools menu).