Where has my storage gone?

In 2019, I installed a 1 TB SSD from Intenso in my notebook.

Speccy still shows "953 GB Intenso SSD", see Figure 1.

When checking this SSD through C: Properties, suddenly only "storage capacity 686 GB" can be determined, see Figure 2.

In addition, the device has starting difficulties, possibly starting hard drive defect?

Is there an explanation for this?

Many thanks in Vo

Then there's probably some partition on it that Windows can't use, e.g. For an operating system

Check that in the hard disk manager of Windows