Google Drive
and Calibre don't mix (Library files) well. It does things to files sometimes to manage versions.
In your case, you had a
Spanned Archive (that is what Export really is) and it messed up one or more chunks of the set.
USB PEN DRIVE or SD card is a good way to sneakernet ( and it can serve as a backup if tucked away afterwards).
Does the old computer still work?? There are other ways to move Huge chunks of data.
USB transfer Cables,
USB drive caddy/clone kits (used to just to attach the drive, not clone) but those require the old system drive be removed.
BluTooth File Transfer (if both system do BT )
File transfer over a Windows Share (OK, but not for an active Calibre session. Timing is too sloppy for that)