Upgrading old laptops from HDD to SSD will leave old HDD's lying on the shelves catching some dust. But if the old HDD's are used as bootable USB-HDD. It can make usage of two or more Operating systems feasible on any computer; which saves the disk space and also not messing up with the default disk present in the laptop. I had a couple of old HDD's. so I buyed a USB 3.0 to S-ATA HDD enclosure. Below image will show you the HDD inserted into a HDD enclosure. I experimented by flashing Ubuntu 18.04 to the HDD and used it as bootable USB-HDD. I used it in a hackthon to learn Docker, Terraform, Packer. It ran smoothly without any issues.
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