Merge is not 'add RAM for disks'
Structural operations
Merging combines filesystems only when tooling supports the layout. Otherwise you are really doing move-data-then-delete-old-volume gymnastics.
Verify both volumes use compatible file systems and are truly adjacent in partition order—not just adjacent letters.
Image both volumes. Merge failures are painful because metadata updates span two former boundaries.
Plan downtime; merging busy volumes risks file locks from search indexers and antivirus scanners.
After merge, validate backups again. One happy chkdsk log is worth ten reassurances from a forum post.
Consider whether merge is necessary—sometimes a mount point into a larger volume is simpler long-term.
Document drive letters that consumers rely on; changing letters breaks shortcuts and some licensed software.