Post-POST, post-BIOS screen, post-GRUB; when I boot Debian with a SD card or USB stick plugged in, one of the first things to pop up on the screen (so quickly that usually I don't even notice) looks a little something like:
[sdX] no caching page mode present
text, text, X starts about halfway through that long list of stuff, the text gets noticeably smaller, and just before the window manager starts, I see the very familiar device message (isn't that what
dmesg stands for, anyhow?)
[sdX] assuming drive cache. write through
So, technically speaking, what's the difference between those two states? What's being cached? What's the page? What's 'writing through'? And what precludes the assumption of drive cache so that the writing through can happen, directly following boot?