Many thanks for all the advice. I eventually purchased a Seagate non SDD mains powered 4TB standalone drive, which I attached to my laptop and partitioned (using Windows) as 2TB plus 4 x 500GB. Added a few files (mp4, avi, mp3, jpg etc.) for testing, and connected the drive to the Fritzbox 7530, and everything worked, after mapping the various partitions on my laptop.
I can see and use files, transfer back and forth, although transferring a film file of +/- 1GB over wi-fi is slow. Watching a film over wi-fi is sometimes fine, sometimes not, depending on who else in the house is doing what simultaneously. Wi-fi here is difficult due to the solid granite walls, so spoke to Zen (ISP) who offer Fritz mesh 3000 repeaters for 7.99 per month, with a promise of a second repeater at no extra charge if one is insufficient to cover the whole house.
Apparently I was 'out of contract' so by signing on for a further 12 months, which resulted in the repeater system being added, and paying 4 pounds less per month! Result.
Just awaiting the arrival of the repeater now. Tomorrow, fingers crossed.