How to make an external SSD into a internal one and use it for OS?

It depends.. Once upon in the time, most external HDDs were ordinary (slow) harddrives with USB-to-SATA (or USB-to-IDE) interface board. Then, some manufacturers (WD Passport is an example) started cutting the SATA interface altogether, and used proprietary connection between the drive and the USB interface.

So, YMMV. If you have the drive and don't mind opening (damaging) it, go forward. If you have found a terrific deal on external SSD way cheaper than corresponding internal SSD - you will take the risk.

There is still a chance that the SSD placed into the external enclosure is slower (being limited by tge USB interface).
 

VampireCookie

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I was asking because I own a unexpandable laptop and I needed more usable storage like recordings etc.

 
OK, then your question is: If I plug external disk into my laptop, can I make it work as "extension" to the main one?
You can keep your work files on that external drive (downloads, music/movies/photos). Assuming you're running Windows 7/8/10, you can include the folders into libraries, and have almost instant access to them. You can even install most of the programs to the external drive (but on some programs, some files still must be copied to the boot drive).

I would not recommend moving your profile directory to the external drive - it will make logging in more problematic. But you can move almost all other system folders there.