So this is what happened:
I tried to open windows media player, naturally to listen to music. But it goes very slow and freezes all the time. I can't play the music.
So I thought, I'd try opening .MP3 files instead. They are located in partition G:\.
My external harddrive (250Gb) is split up in 7 partitions. (E, F, G, H, I, L, M, I don't know why I can't choose J though)
G:\ is normally named "Music", but since my media player freezed, it's called "Local drive G:\" (not sure if it's correctly translated)
I use an ICY BOX for my external drive and it shows a blue light continuously when it's turned on. But now, why I try to access G:\, it stops giving light until this error is shown:

"G:\ is not accessable"
"Error while editing a page"

(again, rough translation)
Every other partition works perfectly.
I tried reconnecting the cables, you never know something isn't fully connected. No success.
I also tried to disconnect the external drive from the ICY BOX and place it in the computer. (the box might be close to broken)
But then I found out I only have SATA connections left... (harddrive is IDE or ATA)
To make sure it's not the harddrive that's broken, I'll try to find an IDE to SATA cable and place it in my computer.
I sure hope it's not the drive that's broken, I haven't made any backups :s .

Also, I can't defragment G:\ as it's not in the list.
If you need any more information, I'm right here.