Yes, you can mount disks as folders using the Change drive letter and paths function in diskmgmt.msc, or the mountvol command-line tool. (But see below...)
However, even on the new disk, it is usually better to keep the files contained within a directory – to prevent it from becoming one big mess if you ever decide to use the second disk for anything else. This can be done using junctions or symbolic links – both will work fine; symlinks are recommended but they're supported only on Windows Vista and newer versions, while junctions also work on Windows 2000/XP.
To symlink your SomeData to a different disk:
- Move the original directory to the new SSD drive. You must delete the original directory.
- Open an elevated command prompt.
Use the mklink utility to create a symbolic link with the same name and location as the original directory:
C:\>mklink /d D:\SomeData Z:\SomeData
(where Z:\ is your SSD and D:\ is the old drive)
On older Windows versions mklink is not present, but the junction utility from Sysinternals can be used to create junctions.