I hope this is the right place for this question.
I have been migrating everything over from an XP desktop (now disconnected from the Internet and from my local WiFi network) to a Windows 8 laptop. That includes Dropbox folders. On examination I find, however, that not everything has been brought over. A rather large number of photo files are stored on an external HDD attached to the XP computer (but not, at least not yet, to the laptop). My photo files far exceeded the 150 or so GB of the HDD on the vintage 2004 desktop, so I kept them on a 2 TB WD "MyBook" external HDD. On the XP box that happened to have the letter M assigned (not my doing, it just worked out that way). All those files are in turn backed up on a second 2 TB external HDD (a WD Passport, not connected to anything except when doing backups).
Some but by no means all of those files appear in the Dropbox folders on the laptop - presumably those that happened still to be active on the computers of someone sharing that particular folder. I can of course connect the MyBook to the laptop, but it will probably not end up as drive letter M (probably something more like E). I could probably migrate all those files to the C:\ drive (1 TB) on the laptop, if necessary (but I would rather leave them on an external, if I have to buy a couple more externals, so be it, today I bought a 3 TB Seagate for $99 at COSTCO). Question arising, how do I tell Dropbox where to find those files?
FWIW the laptop is hard wired into the local LAN (Cat 5 cable from the WiFi router). Other things could be. To be added (now still connected to the XP computer via USB) are the MyBook drive, a Nikon film scanner, a printer (but it is Bluetooth capable, and speed is not really an issue for the kind and quantity of printing I do). The XP box has 5 built-in card reader slots, and those are what take up the drive letters.