Some more input to update the first post.
While they boh have the same named file system, the file system has several versions. And you are correct that the files were not deleted, but I did not know about boot.ini and did the following, and lost my registry files.
When I tried to boot up with win2k after posting my problem , neither hard drive would boot up, when I switch them as slave and Master and back. So I booted the XP hard drive alone with a boootable XP CD and over wrote the system with the slipstreaned up to date, XP. when all was done, I was able now to boot with both hard drives singly without the other. My old win2k now works. In the XP hard drive, although I have the XP installed programs in "Program File" folder, without the old registry they are dead and all my e-mail and identities in thunderbird are gone. But they are sill present in the win2k side. ( I had found in the past a way to import the addresses and identities and e-mail folder sfrom 2k to XP, but I did not document my trial and error method and cannot recall how I did it. I think I searched for Udora files and brought them in that way ( I do not use eudora, it was a way to fool thunderbird. I have not founda tutorial, how to transfer the records from old thunderbird 2k to new thunderbird XP.
I have been thinking to install winXP over win2k so they are able to work together without win2k converting the file system of XP type NTFS to the non working for XP type NTFS downstepped version. Even though both use NTFS they are irreverably altered by win 2k without warning, and create the problem I had faced and still face, with the lower version of NTFS.