Cassette tapes may be obsolete nowadays, but their unique properties still make them almost perfect from the security point of view. Easy to erase, easy to monitor writing of every single sector, and very easy to destroy - everything that SSD devices ARE NOT!

SD cards are small, cheap, light  and fast and they can hold gigabytes of data.
On the other hand, because of their inherent physical properties, they are far from perfect from the security standpoint. We explained almost everything here:

This device solves some problems of secure data management on SD cards:

Then why bother with obsolete cassette tape technology at all?
Thanks to the following properties, cassette tapes may still be more convenient than SDs:
1.)  Easy to erase.
2.)  Easy to monitor and control every sector of data.
3.)  Easy to destroy.

The last property may be crucial in a spy scenario, where Alice must quickly destroy the codes when chased by Mallory or simply to prevent the dumpster-diving recovery of data  by Eve! The PET-based tape melts at cca. 250°C and boils at 350°C, which is more practical than minimum 1300°C required for SSD.

Tape speed variation, wow and flutter are the main problems of cassette tape technology. This device uses a different approach - instead of precisely controlling the tape speed (which is very expensive in any aspect), the tape speed is constantly measured, and system clock frequency is varied accordingly. This way we can use any below-average tape deck, since this device can compensate for wow-flutter variations up to +/- 10% The only requirement is 200Hz-3000Hz frequency span which is achievable even with a dictaphone microcassette running at only 2.4 cm/s.