I try put gentoo on usb 8Gb using a minimal cd .
I read and follow steps from gentoo site how to install it.
I use grub-install --root-directory=/boot /dev/sda
I use grub --config-file to test grub.conf , but i got error 28 .
Can i put gentoo on usb flash ?
How i make working and remove this error ?
Thank you .

Did you take a look at the Gentoo documentation?

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I don't want a live gentoo usb . I can use a "Universal USB Installer" from windows...
I want gentoo usb like a any hdd instalation.
It's not possible ? Maybe i lost something when i read it.
Thank you .

I use gentoo on an external usb disk, so it is possible. Its probably something about the partitioning on your disk, that grub can't fit.

Try it using LILO instead... ! just a guess

Maybe it's that the stages from your local grub version in your HD and the grub version in your USB don't match. That can cause odd errors. If you are going to your the stuff in the $USB/boot/ directory to boot you should use that as --root-directory probably. Or, just make sure that the grub versions in both devices are the same, then you shouldn't need to worry. I assume both installs do use the same architecture as well...

I am no big expert in grub though. If you can't find help here there's a huge thread about grub problems in the official Gentoo forum.

If you have other harddisks in that system the kernel will not recognize your stick as sda. Maybe thats were the problem is?
Can you boot the system with supergrubdisk to find out?

Yes i read it.
Now i solve with another distro... but maybe another geentou user have same problem .