Why can't I create a Volume as large as the Partition?

[Guido Lißmann]

Hi, new guy to the forum, long-time VeraCrypt user, and TrueCrypt before that. Never had a problem like this one, hence me asking for help here.

Situation: New computer. Several SSDs and a 6gig HDD which is for making backups of the SSDs.
I have a 2TB SSD, on which I created an encrypted volume, full size, no problems.

The HDD, meanwhile.
Initial trouble because Windows, during installation, insisted on slapping a system-reserved partition in there withouot asking, and slapping an MBR on there. Which wouldn't let me use anything more than the first 2TB, of course (why do I feel reminded of "nobody will ever need more than 640kb...").

I tried creating an encrypted volume in those first 2TB, but no matter what I tried, VeraCrypt would not let me make it larger than 1TB. I couldn't even enter anything, not even anything smaller.

Over some trials and tribulations, I was able to nuke that MBR and put a GPT in place instead.
I put a few partitions in place, and tried to create an encrypted volume.
Same problem as before (see screenshot).

I tried it with an unformatted partition, a formatted partition, and just for kicks, I nuked one of the two 2TB partitions I had made, just to see what would happen. Nothing helpful happened.

I never had this problem before. I created a 2TB encrypted volume on that aforementioned SSD only yesterday.
I really doubt it's a bug, I'm pretty sure I either have hte dumb, or am missing something else. Quite probably something quite simple.

What can I do? My plan was/is to have two encrypted volumes, and two non-encrypted volumes on that HDD. So far, no data has been put on it, so nuking the partitions I made is entirely an option. I had an HDD with two encrypted and one unencrypted volume in my old machine, all coexisting peacefulls, I'm not really on ground that's entirely new to me.

Any help will be greatly appreciated!

NOTE: yes, it says 1.91 TB in the screenshot. I partitioned 2,000,000 MB, which is the size I wanted, and since I never took the numerical mathematics final exam at uni, I don't think in base 2, so I count it like the hardware manufacturers do.