Q: how to install Windows 10 on unsupported mac

I am currently giving up and trying to install Windows 7 instead hoping that it will go fine

I don't believe you can install Windows 7 either with Boot Camp on El Capitan:

• A Microsoft Windows installation media or disk image (ISO) containing a 64-bit version of Microsoft Windows 8 or later*

From: How to install Windows using Boot Camp - Apple Support.

Windows 10 is simply too new for your Mac, and Windows 7 is too old. It's quite a predicament. If you really need Windows, you would either need to obtain a copy of Windows 8, or downgrade to an earlier version of OS X with a Boot Camp version that supports Windows 7.

I installed windows 7 on my mid 2011 21" iMac and upgraded the install first to windows8 and then to 8.1 and then to 10 mind you when the device is not supported there are no win10 drivers for you so you will have to make do with the older bootcamp drivers so not everything will work

Install W10 using a physical DVD and disable Windows Updates. The forced updates of W10 cause failures with GPU drivers.

whyisappledoingthis wrote:

 

what would be the difference between using a DVD and a USB drive to install win10?

A DVD boot is different from USB boot, for example, the drivers being loaded to support either media are different. Also see the El Torito standard.  A USB HDD is not supported, but a USB Flash drive is supported. With USBs larger than 32GB, new issues develop, for example, Windows installation on external devices becomes possible with 64+ GB flash drives. Anti-piracy issues crop up. To make money, M$ started Windows-to-Go.

All Macs support USB boot using GPT/EFI, but Windows uses (up to W7) legacy BIOS/MBR boot.

To support Windows booting on Macs prior to 2012, a USB is not used because the EFI code on these Macs does not support MBR devices as boot devices and the device discovery via EFI does not expose devices properly.

2012 Macs support boot using either CD/DVD or USB or a combination.

2013 and later Macs (up to 2014) support USB (or external CD/DVD for specific models, like MacBook Air and Apple Superdrives). The late 2013 models are the first UEFI models (EFI2.0+) which support EFI boots consistently across most OSes and eliminate the need for CSM-BIOS (BIOS emulation via EFI on preUEFI computers).

2015 and later models stop supporting any legacy BIOS/MBR and will only support EFI (W8+, W10). W7 EFI is fairly unstable on Macs, with Sleep/Wake/Hibernate issues.

Some USB manufacturers honor boot ability requirements, but generic no name USB drives do not, which is another issue with USB. Apple eliminated the whole USB method in 2015 and later Macs. Windows Repair can be problematic on some models without a Windows Installer.

Despite all Apple efforts to 'kill' CD/DVDs, an Apple Superdrive is till sold.

Recovery is another area. Apple supports network boot. W10 was expected to allow booting similarly from Microsoft servers, but they stopped at just providing ISOs. W10 in 2017 may change this.

A 2011 Mac also supports 64-bit Windows. For example, see Install Windows 7 and earlier on your Mac using Boot Camp - Apple Support for Macs that can run W7 64-bit.