Yes, you bumped against a drive letter limit.
In disc management, you can mount a volume (drive or partition) into a NTFS folder instead of assigning a drive letter.
I am doing this with a second internal HDD. Instead of using drive letters like D:, E: F:, I access the partitions via a folder C:\mnt like this:
D: -> C:\mnt\HD2_part1
E: -> C:\mnt\HD2_part2
F: -> C:\mnt\HD2_part3
Saves a lot of drive letters