Rather than put the SD into the Mac to transfer the files, you could transfer the files from the Mac to The Pi via your network. Look at SCP/(s)FTP protocol for this.
SCP is built into the Mac but needs to be done from the command line, FTP may be easier if you use something like FileZilla but you’ll need to go into the OSMC App Store to enable FTP first. Normally I recommend SCP over FTP as you don’t need to enable anything on OSMC but I don’t know of a nice graphical SCP client for the Mac.