Hi everyone and happy new year !
I had a G550 for Christmas (cheap present, isn't it ?) and the drive is weirdly partitionned :
- a first W7 partition around 200 Mo (ok)
- C is almost 450 Go (Why ???)
- D (the Data space) is around 30 Go
- and the OEM parition (to restore the system) around 14 Go (ok)
I would have liked :
- C around 50 Go
- D around 400 Go
And all left for the system
I read here and there : be carefull, oh my god, don't touch, do it, do that, .... Well, here is what I did and it works just fine :
1. I use Gparted (use Google or Bing to find it), an open-awesome-source software
2. Reboot the G550 with it
3. Change the C partition to 50 Go, delete the old D one, change it to 400 Go and Apply the changes. DON'T touch the first one (W7 +- 200 Mo) and the last one (the LENOVO_PART, nyfs, 6.78 Gio occupied / 7.97 Gio free with a diag flag One)
4. Reboot the G550. W7 make a "quick enough" chkdsk control on the disk
5. Now use the One Key Recovery Tool to restore a brand new W7 on C. In less than 30 mn your computer is available with a C around 50 Go and a D around 400 Go.
During the One Key Recovering process, I hope the D drive is not formated, which I didn't control as I didn't think to put files on it. Try it and tell us... That is what Computers World is about : TRYING ! I already have lost all my datas, my mother, my brother, my dog.... Don't be afraid, you came naked, you will left us naked !
ps : Back-up all your datas on another drive before going there if you don't want to know what naked means 
Oh and I know what DOS is, Do you ?
Disk Operating System for the newbies
