Windows stores the EFS encryption key, encrypted, in the registry... for domain logons, I assume it's stored on the domain controller. For non-domain machines, you'll probably need to make sure that all machines have the right credentials, and perhaps SIDs as well.
The problem with NTFS files is they have the concept of "ownership" attached to them. If that ownership is attached by a central authority (domain controller), then switching disks among domain members shouldn't be a problem. But when you mount a "foreign volume" from outside the central authority, then who owns these files? ...the Default User?
Should you even be able to mount a foreign volume? If so, then who takes ownership of the Default User's files? In this weird case, I "think" the Default User would be the local administrator since the creator of the original domain account to which these lost files once belonged to would not be available on a foreign, non-member host. The other possibility is that there is no defined Default User; therefore, you can't mount the foreign volume.
I have strongly discouraged users from formatting their USB flash drives with NTFS directories if they are taking them outside their Windows domain for fear it might create ownership problems down the road. Even if those flash drive files are owned by the Everyone group, it's still the Everyone group for that specific domain, not the entire Windows world.
If there is a safe approach for defining NTFS ownership on portable (foreign) disk volumes, could someone step forward and explain this? For security reasons, I don't like users using FAT volumes, but for portable disks, I'm not sure how to get NTFS ownership to work.