Sudden loss of writing to recording files

After a recent hard drive replacement, with no cloning of my old drive possible (but a 15-track Reaper project, along with other important stuff was saved to a server, for retrieval), Windows 7 was re-installed, and I downloaded up to Reaper 4.51 (from 3.5, I think) to "house" my project file. During an editing session, I decided to record some additional "riffs" to an instrumental track, with the record function set to "punch-in..time selection" mode. When I clicked the record button, a window popped with "Can't write to recording files--does the directory exist?" I tried the other 2 recording modes, with the same result; spent 2 hours trying to troubleshoot in the Reaper manual, with no luck, and an additional hour scouring the Forum. Anyone able to tell me what this means, and is there a fix? I'm only able to edit the existing files, but record function is lost! Need help fast!