Research before Buying. (A) Many of the newer USB HDDs have started to incorporate the Sata -> USB circuitry inside the HDD therefore do NOT lend themselfs to puting into a SATA port. (B) many of the Newer external HDDs have moved to the mSata drives. If it is SATA then for a laptop designed to interface with the standard SATA, But some have adapted the smaller mSATA interface.
Interchangability between mSata and SATA depends on input voltage the mSata (I think) uses 3.3 V while Sata Uses the +5V. If this is the case a simple cable adaptor may not work even if the laptop had space. More on this.
While the 3.3, 5 and 5V are part of the sata Specs, that does NOT mean that they are always present (primarily laptops). Normally present on SATA Power connector for desktops, BUT for laptops space is aften primariy, so since lapton is designed for say 2.5" HDD they do not route the +12V and may even omit the 3.3 V. if the Laptop is designed for mSata and a NEW 1.8 in HDD it may only require (and supplied with) 3.3 V.
Bottom Line, would recommend buying a INTERNAL HDD designed with the correct interface.