Reformatting former RAID HDD for repurposing.

I had a near disaster with my ReadyNas 516 6 x 4TB. I had a single HDD failure and promply ordred a 4TB replacement. Unfortunately i missed the next day delivery and had to go away to my work for three weeks. During my final week away the system expeienced a power failure and following reboot a 2nd failure occured. Fortunately I took the libery of making a complete fresh back-up to my 2x10TB lacie solution before I left. I wasnt aware that there is no recovery from second HDD failure using raid 5.

I purchased a second 4 TB HDD replaced and carried out a factory reset. After a 10 hour resync I was ready to reload my data,

During resync i recived a message that one of my HDD had errors, 22 bad secctors, i have now replaced that with a third 4TB HDD.

That was easy Hot Swap and 9 hours resync, but at least i now have complete peace of mind.

This brings me on to my question. I want to reformat the degraded 4TB HHD for non-important use. I have put it in a USB caddy connected to my windows 10 PC. It had three active partitions. I removed these to create a single parftion using disk manager, but it only reconises less than half of the 4 TB. I have tried using diskpart and clean to no avail. I am now wondering if my caddy is too old for a 4TB HDD so i have ordered a more up to date oned with USB 3 to see if that helps. in time i want to repace the three remaining HDD with new and repurpose those too.