The Harddisk# and DR# numbers both refer to a Drive - this is normally a physical drive (HDD, SSD, USB) - each Drive may contain multiple partitions each of which may have a drive letter.
The Harddisk# number matches the Disk# ID number shown by the Windows Disk Management utility. Harddisk# numbers are allocated sequentially in real time as disks are detected or connected. For internal HDD's the allocated number normally remains tied to a particular physical drive - assuming internal cable connections remain the same. For USB removable drives things get more confusing - the next available # number is allocated during connection (filling gaps if present) - if a drive is disconnected the number is removed leaving a gap - once a drive is disconnected there is no record of previously allocated numbers. Therefore matching a historical Harddisk# ID to a physical internal drive is possible - but for USB removable drives the # number only implies how many drives were present in addition to the fixed drives.
The DR# number is also dynamically allocated but follows different rules - numbers are allocated sequentially for all fixed and removable drives at detection or connection - however numbers are not reused while Windows is booted. Each time a removable drive is disconnected and then reconnected the DR# number increments. The numbers are NOT tied to USB port numbers as implied in the previous posting.
Initially the Harddisk# and DR# number will match - as both assign the next available sequential number. But active use of removable drives (such as repeated use of multiple USB flashdrives) will cause the DR# numbers to increment far above the associated Harddisk# number. The longer Windows remains booted the higher the DR# numbers can rise - only when Windows restarts does the DR# number allocation get reset - the DR# number is not reset during User Logoff events only at a Windows Restart.
For physical internal drives the two numbers generally remain fixed and identical. But for USB removable drives there is no way to tie a DR# number to a physical device - a removable drive may have been allocated many different DR# numbers even during a single booted session.
The Microsoft SysInternals utility WinObj.exe displays the currently assigned ID's via the Device section of the tree (requires "Run As Administrator" privileges).