Why is my hard drive showing as invalid when in an enclosure?

Hi,

I have 2x 2TB 3.5" hard drives.

A is about 90% full, B is about 99.5% full.

I purchased 2 hard drive enclosures (both exactly the same) and installed my drives into them.

Drive A shows up via USB

Drive B doesn't. Disk Management shows it as invalid and my only option is to convert it to a basic drive which will wipe the contents (I am not prepared to do this).

I plugged drive B directly into my PC and it shows up fine.

I swapped the drives around enclosures to make sure the fault doesn't lie with the enclosure and the same results occur.

Any idea why this could be happening?

The only difference in the drives is how full they are, but this can't be the issue, can it?

Thanks in advance you smart smart people.