I have an Acer laptop - 1tb HDD ,18gb SSD windows is not installing in HDD ,only installing in SSD .

Solution
I suppose there must be a setting in the BIOS that can switch the SSD to being either recognized as a cache or as an independent drive.

M.2 SSDs used as cache are 22mm wide by 42mm long. They are referred to as "2242 SSD" as opposed to the more common "2280 SSD" which are 80mm long. I would buy a 128GB 2242 SSD and replace the 18GB SSD. That way you can install Windows so that it can boot a lot faster compared to booting up from the hard drive. Programs or a couple of games that you use play a lot can be installed on the 128GB SSD.

A 128GB SSD will probably show up as having 123GB capacity after it has been formatted. You should never use more than 80% of a SSD's capacity. The general recommendation is that 20% of the capacity should...
How was this system setup originally? You can't just make an SSD run as a cache to a hard drive, hybrid drives are setup as a single drive and use their controller and firmware to work. Windows should have no issues installing to any drive you tell it to, as long as that drive is in the right format for Windows. Meaning you wipe the partition and have it create the one it needs.
 
I suppose there must be a setting in the BIOS that can switch the SSD to being either recognized as a cache or as an independent drive.

M.2 SSDs used as cache are 22mm wide by 42mm long. They are referred to as "2242 SSD" as opposed to the more common "2280 SSD" which are 80mm long. I would buy a 128GB 2242 SSD and replace the 18GB SSD. That way you can install Windows so that it can boot a lot faster compared to booting up from the hard drive. Programs or a couple of games that you use play a lot can be installed on the 128GB SSD.

A 128GB SSD will probably show up as having 123GB capacity after it has been formatted. You should never use more than 80% of a SSD's capacity. The general recommendation is that 20% of the capacity should be free to prolong the life of the SSD. The free space basically reduces the number of writes / re-writes in a specific sectors. Each sector on a SSD can only be over-ridden so many times before it simply fails. Statistics for modern SSDs generally points to the fact that SSDs can be used for up to 6 or 7 years before they begin to fail.

Here's a example of a 128GB 2242 SSD selling for $70

https://www.amazon.com/ADATA-IM2S3334-Industrial-Grade-Internal-IM2S3334-128GD/dp/B074JMMW4N/ref=sr_1_2?s=electronics&ie=UTF8&qid=1512767055&sr=1-2&keywords=m.2+2242+128gb+ssd
 
Solution