Disk Management

OK, just to confirm, the 150MB of free space is on another partition? If so, you can use a disk management tool (I personally like GParted) to resize that partition down to 0, and then resize your 750MB one to take up the free space. It won't affect any of the data on your larger existing partition.

Just out of interest, why the separate partitions in the first place, and why merge them now?

Ok the thing was, that I was installing Ubuntu, but I made a mistake and was installed on the Documents HDD, but fortunetly, just on the free space, now I erased the partition that Linux created, and appeared free space on the disk, but is not part of the Documents partition in this moment (it was before installing linux), so, when I clic on the documents partition, I can extend my volume with the free space that is available on the disc, but I'm not sure If that will erase all the data that is on the HD... will It???

But remember that any operation like this is potentially risky (I have - very rarely - lost data when resizing a partition). If there's anything that you can't afford to lose make sure you have a backup of it somewhere else.

But everyone does this anyway, don't they?   Hard disks are not infallible; should something go wrong, just in the normal course of events, you could lose all your data in an instant.

Best answer selected by Xtrastor.