Once you have copy/verified the transfer of the bitcoin folder and it's entire contents onto an external hard drive with enough room, create a BAT file in the "BIN" FOLDER/DIRECTORY called
bitcoin.bat using any notepad style program (not "office" or "open office" - I use
NOTETAB PRO)
add the following line of text:
bitcoin-qt.exe -datadir=%~dp0
save, then double click on the
bitcoin.bat file. The above line of code will tell your program that everything it needs is in that folder.
Having said the above, if you were starting from scratch, unzip the contents of the version 21.0 zip file into a new folder, then do the above steps.
You can add the
wallet.dat prior to first running the program, or, at any point after you shut the program down.
AND MAKE BACKUPS OF EVERYTHING & ANYTHING BEFORE YOU TRY ANY OF THE ABOVE!