Why does turning an internal hard drive into external cause problem?

[Vorpike]

Why can't I simply take an internal hard drive, plug it into a SATA to USB and plug it into a USB port? The hard drive isn't readable and apparently the only way to make it so is by formatting it.

 

[ShadyHamster]

Could be an issue with the Sata to USB converter.
Do you have another HDD to try?

 

[John7128]

You can and that should be fine. First things coming to mind is the partition type. What computer is it coming from/OS and which computer do you use it on/OS?
Was this internal hard drive with an OS?