我才知道 host OS 和 guest OS 不能同时挂载一个 usb disk，但是大错已经酿成。
当时我正在 host OS 里从 host OS 的硬盘移动一些文件到 usb disk 上，然后又在虚拟机里加载 usb disk 给 guest OS，结果，usb disk 就自动从 host OS 里卸载了。卸载时一些文件正在移动，突如其来的卸载造成这些文件未成功写入 usb disk，但源文件却已经从 host OS 的硬盘上删除了。
Host OS 是 Fedora，这教训给我一个感觉，Fedora (或者说是 GNOME，我不清楚该责怪谁) 的文件操作机制不够可靠——移动文件时，应该在收到目的文件写入成功信号后才删除源文件。
The following command sets the timezone to British Summer Time (BST, UTC + 1) or Greenwich Mean Time (GMT, UTC)
cd /etc
ln -sf /usr/share/zoneinfo/Europe/London localtime
Or, update server timezone
tzselect
update server system clock
ntpdate ntp.org
I recently installed Windows XP in VirtualBox, and did all necessary updates, including IE 8. However, IE 8 kept crashing. At first, I thought it was a problem with VirtualBox or its Guest Additions. The same problem exists in kvm. Then I thought it was a general problem of running IE 8 in virtual machines.
But inspired by a comment of IE 8, I downloaded IE8-WindowsXP-x86-ENU.exe and re-installed IE 8. The problem has gone. I never expected I have to install IE 8 twice to get it working properly in a newly installed (clean) Windows XP.