I have Problem while installing a 2nd OS (W7) on a laptop with W8.1
 
I am trying to have 2 OSs on the same HDD. My laptop is an Acer V3-572-5217.

I partitioned the 1 TB drive into 2 partitions, on the first is pre-installed W8.1 and in the othert I want to install W7.

I set an USB drive with RUFUS program. Inside that program I configured the format style as GPD because the HDD has GPD format style.

Now, when booting from the USB (With W7 Ultimate 64bit) (UEFI is enabled), the files charged and the screen of "Starting Windows" appeared, but then a line on the top of the screen appeared like the screen was cracked, the line disappeared and then the laptop froze.

I dont want to COMPLETELY clean the HDD because I would lose the original Windows 8.1

Please help me.
I have done this more that once. Here are method(s) I use.
A). Use two drives with a boot manager that let rs you select either drive.
OR
B). Obtain a Windows 8.1 install DVD
OR
C) Do not use UEFI.

IMO, your job is easier if you have three hard drives to work with.
The drive in the computer, an external backup, and a spare drive used for temporary configurations.

Your suggestion is a popular topic. The answer is not what you want to hear.
See here:
How to install Win7 on my Win8 laptop?
Quote

Once you install windows 8 on a system it will not allow you to install a prior OS like windows 7, this is because of the way the file system is layed down on the HD.
Windows 7 uses a NTFS file structure where as Windows 8 Uses a GPT file structure.
The data is layed down and located on different parts of the drive. That is why windows 7 cannot install due to the Gpt format of the drive. As it is an older operating system and does not understand GPT format.

If Windows 8 was preinstalled on your laptop you will need to get into bios on your laptop and turn off UEFI Secure Boot and enable "Legacy" boot.
Installing a new OS can sometimes be a complicated process especially if windows doesn't have network or wireless drivers for your particular network card baked into the OS. What I would do is not TOUCH your current HDD and just give it a go with the new SSD and post back here with any problems you might have.

...
get an external hard drive, install windows 7 in it, then next time you boot up and want to boot into windows 7, with the hard drive plugged in, start up your computer and it boots into windows 7 or if you having troubles with that, just select  the hard drive from your bios to boot up windows 7

Have you done that? Ever? How?