How can I know if a disk is SSD or HDD in OS X?

Apple Menu > About this Mac > System Report > Storage: click on the volume in the right column - Medium Type will show rotational or SSD as appropriate:

rodion15 wrote:

Thanks a lot. It seems when it's an external drive it doesn't show this (at least with the USB ones I tried), doesn't show this neither under USB nor under Storage in System profiler.

If as may well be the case it does not show the Medium Type for external disks then probably the best option is to run Disk Utility. In Disk Utility you can select the entry for the drive (not the volume) and try Get Info. I suspect this will for the same reason not be able to show the Medium Type but it should show the model number for the drive. You can then search for this in Google to check whether it is an SSD or traditional hard disk.

rodion15 wrote:

Thanks a lot. It seems when it's an external drive it doesn't show this (at least with the USB ones I tried), doesn't show this neither under USB nor under Storage in System profiler.

No, it won't show if you use a USB drive, but you can pick it up and look at it easier than when it is an internal

the terminal app can tell if it is core storage or not , copy and paste:

diskutil cs list

Core Storage is a logical volume management system on OS X that was introduced by Apple to OS X Lion