There are a couple of CLI tools that can be used to benchmark an HDD (external or internal). These are dd and hdparm. However, for Debian Linux, there’s a great GUI application that performs Disk Read speed benchmarking and both Disk Read & Write speed benchmarking if the drive being tested is unmounted. The dd command is used to monitor the writing performance of a disk device on a Linux and Unix-like system. An example for testing an internal SSD would be: 

$ sudo dd bs=1M count=256 if=/dev/zero of=test