Answer:
The HDD MUST be mounted as RW!
http://www.knoppix.net/wiki/Rescue_FAQ
Hi there and wormest regards from Vienna!
I've a little problem here! Trying to boot up my system I got the MSG "HDD Full" and the boot stopps...
That's why I decidet to use Knoppix to get rid of some Folders.
The Problem is that the specific folders I need to delete belong to root/root. I mamaged it to set a root-pw and so on, but that seems to be root@Knoppix and that's no help so far!
Anyway I need to del files that belong to the root of the local filesystem on the HDD (sda5). Any ideas!? ^^
Answer:
The HDD MUST be mounted as RW!
http://www.knoppix.net/wiki/Rescue_FAQ
You might also find that, if the disk is NTFS, it is very unsafe to write to it at all with Knoppix. Some people do think that current versions of Knoppix can safely write to NTFS. But some people have been saying that for almost every version of Knoppix, and all previous versions that some people thought were safe to write to NTFS with were eventually found to have problems that led to hard disk corruption. Some of us are not yet convinced that writing to NTFS is safe, and would use other recovery techniques. At the very least I would have suggested using Knoppix to back up important data to another system (although I guess if there was any "important" data it would already have been properly backed up).
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Verifying of md5 checksum and burning a CD at slow speed are important.

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