[ubuntu] Cant write to external usb disk, hardware or software problem???

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http://ubuntuforums.org – Today i started having a weird problem with my external usb disk, i copied some files to it, and the first bunch copied without a problem, but then it unmounted itself halfway through, and now everytime i try copying something to it it starts copying the file but then it unmount s itself at middle of copying it (usually at exactly 64,4 megabytes for some reason). I can still read all the disk contents without a itch, but when i copy something it always ends unmounting itself. Since when i want to copy something to it i run nautilus as root (it´s owner is root) so i doubt is any permissions problem, the disk also has still much available free space, so it cant be that either. My question is how can i diagnose if the problem is software (messed up filesystem) or hardware (damaged disk sectors), or some other kind of problem and fix it if possible. The disk is an Iomega Prestige Desktop Hard Drive 1TB formatted in ext3. Personally i´m not very worried about losing the disk itself, but some of the data on it is very precious to me. Thanks on advance for any help. (Hardware)