So my question is if I connect that 7200 RPM desktop PC’s hard disk to laptop using USB-to-SATA cable, will it increase speed performance? As the disk on USB-to-SATA cable will use USB’s power, it may reduce the performance. Does anybody experience doing like this?
Speed of the external drive won’t really be impacted due it getting any power via the USB port. The speed will be impacted by the USB protocol being used (2.0 versus 3.0) as well as the USB-to-SATA bridging circuitry itself. How bad/good that would be depends on many factors.
First, will the USB enclosure or cable be USB 2.0 or USB 3.0? Since you have a Dell Latitude E5420 and that system only has USB 2.0 ports on it, a USB 3.0 connection to the external drive should be pretty solid speed-wise but not so much with USB 2.0. That said, checking the specs it seems it has one port that is a combo USB/eSATA port which is a potential solution; eSATA will give you better performance right away since that would be a direct connection to the SATA bus on the laptop itself.
But if you want to try using USB 2.0 that would still hinge on the USB-to-SATA bridging board itself. And that can be a toss up. Some USB-to-SATA bridging boards give good speed, others not so much. Sometimes it’s an issue with the USB aspect of the bridge, sometimes it’s the SATA aspect, sometimes it’s a combination of both.
My advice to you is if you can find a decent, low-cost USB 2.0 or eSATA external enclosure it’s worth a gamble to see if it helps your speed situation. And I would definitely recommend going with eSATA over USB since that would be a direct connection to the SATA bus on the laptop itself; no USB 2.0 or data protocol bridging bottlenecks.