[ddouglass]

I think you are mixing two problems in this. First is importing them into Hoster and second is copying the KMA files to another drive for back-up. Am I right?
If this isn't the case you are going to need to explain step by step exactly what you are trying to do.
These external drives are they formated in file system NTFS or FAT? Right click on the drive, go to properties and toward the top of the page it will say "File System:" then what type it is.
If they are FAT then your problem may be the limitation of how many files\folders can be created under one folder (directory is the same thing).
Even NTFS has a limitation on how long the file path name can be, so if you are trying to put these files too deep into a folder then Windows can't do it.
In Hoster when you are importing these files, what error message do you get? Is it always stopping at the exact same MP3+G or is it random?
You say they import ok if you copy them to the C:\ drive. If you copied (not Move) batches of them to C:\, imported them (creating KMA file on external), then delete that batch from C:\ (since you still have them on another external you won't lose them) and repeat for the rest then you should be able to get around this problem.