OS 10.9 can’t operate with that new file system. Second, the update to 10.15 surely reformatted your hard drive to APFS.

The new firmware probably won’t allow your computer to run under 10.9, even from an external drive. (Space on drive isn’t a reason, since you can install a larger one.) First, when you updated to 10.15 you likely updated your firmware too. The suggestion to make your computer dual bootable, now with both an OS 10.9 partition and an OS 10.15 partition, probably won’t work, for two reasons. There’s no way to install an older OS on the current computer…regress back. BUT, regardless, if you need FCP 6 to work, you need to start from scratch on it…install an older OS and reinstall the software. If you need the newer OS for some things, then you need to have two partitions on this computer…or two separate computers. So…alas, if you want to run the older FCP 6, you need to run an older OS. You MIGHT be able to partition the HD now to do that, but if it’s very full, it won’t really work. This might mean wiping the drive and doing everything from scratch…making the two partitions, installing the different OS versions on each partition, and then installing your apps. The only way to get FCP 6 to work on that laptop now is to partition the hard drive and run an older OS on it. Or at least a partition of a hard drive with an older OS, that you can boot too when needed. If you rely on FCP 6 and need it to work, then you need to run it on a machine with an OS that supports it. Sorry, but FCP 6 is over 10 years old…and with any software that old, the later the OS version, the more chance of that old software not working. So I’m pretty sure that FCP 6 working in the previous OS, High Sierra, would be a pipe dream. And Apple officially announced that FCP 7 would not work in Mojave, period. Many components of FCS 3 (FCP 7) broke with 10.9.5. Sorry, the last OS to officially support FCP 6 was 10.5.

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