Recovery & Knoppix
Dirty means that there are filesystem errors on disk.
Is it ntfs or FAT32 partition?Normaly,you can fix it by the help of windows xp instalation disk using recovery
console typing command chkdsk /r.(Or better, put disk in some other computer with XP).You must know Administrator password. chkdsk /r could fix the system.
You can also try to save data with Knoppix.If you access to disk from Knoppix Desktop as normal user,you can't write to disks(read-only file system).You have to open console and type su.After that, type mount /dev/hda1.And on the same way mount your external disk.
Now type command mount to see where they are
Usually acces point to that disks is folder /media.
Now type mc .By the help of that program You can copy all files from one to other disk.
I'm not sure,does knoppix have some tools to fix dirty ntfs filesystem,but for fat32 there is command dosfsck .
Is your external disk also ntfs?If yes,than you could have some problems to copy data.It's better to make backup on FAT32.