Acer has been my computer of choice for personal purchases recently (sorry Dell). Regardless of the type of computer I buy, I always, ALWAYS reinstall Windows on it. I do this to remove all the preinstalled crap(ware) that comes with most new computers. I like a fresh, clean install on my computers (along with solid state drives). Recently, I purchased a new Acer Aspire S3 391 and discovered something interesting during a clean installation of Windows 8.1.
For Windows 7 and Windows 8 installs, I generally try to use the free Windows 7 USB/DVD Download Tool to create a bootable USB drive. Quick and easy. However, I soon discovered that the only way for the Acer ultrabook to recognize a bootable USB drive to install Windows was to set the BIOS to Legacy Mode instead of UEFI. Basically, UEFI is short for Unified Extensible Firmware Interface and is the standard firmware interface for newer computers and was designed to be the successor to the BIOS. The main advantages of UEFI are the support of GPT (GUID partition table) drives, which allow you to use drives larger than 2TB, Secure Boot, and a faster boot-up process.
Making the change was simple enough and then installation runs as normal. Unfortunately, there is a 'gotcha' if you make that changes. If you reinstall Windows 8/8.1 with the BIOS set to Legacy Mode, when you change back to UEFI boot mode, it says "no bootable devices found". If you change the setting back to Legacy Mode, it works as normal.
While UEFI is not required, if you are a geek like me then you probably want to make use of all the capabilities in the newer hardware. After a little research, I discovered that the Windows 7 USB/DVD Download Tool always formats an existing partition with NTFS. Unfortunately, FAT32 is required for booting up in UEFI mode. If you want to use UEFI, the easiest process is to install Windows the old fashion way using a installation DVD (this is alternative to trying to convert the drive into a GPT drive). After installing Windows 8.1 from DVD (which you can use the Windows 7 USB/DVD Download Tool to create the DVD), I was able to enable UEFI on my Acer laptop and boot into Windows.