Thank you for your question and interest in the 5580.
It's very rare for Dell to adjust any system firmware in order to limit a system to below NTFS or GPT standards. I have no reason to believe that a firmware limit has been set on the 5580. Even when the user's guide says 512GB max.
I pulled up my internal customization page and my internal parts replacement page and each reflects maximums of 512GB drives. This means that Dell simply hasn't, and currently does not sell the 5580 with a 1TB M.2. This does not mean that the firmware does not support drives that are over 512GB.
I suspect that Crucial has tested this by physically installing a 1TB M.2 drive and found that it worked for them. Now can I guarantee any one 1TB drive will work? No. You'll simply have to try it at your own risk. It's my firm opinion that most 1TB M.2 drives will work with the 5580 however.
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