What should I do if my Btrfs partition does not mount?
The full list of btrfs mount options should be here https://btrfs.wiki.kernel.org/index.php/Mount_options and other things may be useful also, such as noatime,nodatacow (fixed a kernel bug for me giving me a chance to copy my files). Add it to your grub.cfg / menu.lst, or type it in when booting.
Is it possible to recover information from Btrfs?
To some extent, BTRFS is capable of recovering some of the information due to its inherent redundancy, we can do this by scrubbing. If the journal is inconsistent, BTRFS will not let us mount the drive with write permissions.
How to throw away incomplete transactions in Btrfs?
In order to throw away the incomplete transactions we use btrfs rescue zero-log. In case that the superblock is affected, we will not be able to mount at all. The superblock is the root of the filesystem tree and contains the information that the operating system needs to mount the unit.
Is it safe to scrub a Btrfs partition?
Scrubbing is considered safe and is usually the first thing to try. This will fix as much as it can, but might not be able to fix all the issues. Try again to copy to safety whatever had errors before. Probably we will have fixed many files and maybe even all of them.
What does it mean when Btrfs says transId 1823?
It means that btrfs wanted transid 1823 (That was on the journal) but on the disk it saw 1821. So the disk was 2 transactions away from being in sync with journal. I personally would risk a brtfs-zero-log here just because its only 2 transactions.
Is it safe to run btrfsck on sda5?
Again I told you bout being safe thing, if its your only data and its mega critical (shame on you), and I would do the below operations first to be safe. Running a btrfsck /dev/sda5 (which by the way just does a readonly check so its completely safe, its only btrfsck options that you have to worry about) will also show you those messages.