No drive letter for an internal drive

My system has two slots for M.2 drives and a connection for an internal drive. Both of the M.2 slots are in use. No problem there.
I also installed a 256 GB internal SSD using the connection for one. Disk Management lists it as Disk 2 and the events for the drive show that I installed the drive on January 4 (this year). It says it has three partitions, all healthy:
- EFI partition
- 1 GB partition
- 237 GB partition
For the properties of Disk 2 the Volumes tab says the Partition Style is GPT. I see no option to format the drive and I assume I do not need to; I probably did that already. The drive does not have a drive letter. I cannot assign a drive letter to the drive (partition). How can I use the drive?