I just had the chance to test things on an old MacBook (OS X 10.6.8).
Having the OP-Z connected via USB, I turned it on while holding the screen button. After a few seconds, I was able to access the OP-Z partition, being able to read and write to it without problems. The OP-Z partition is formatted in FAT format, which can be read and written by OS X.
Having this observed, I’d say that your OS X Yosemite should work just fine.
In your case, it seems like the partition table got damaged somehow, which might be the reason for the error message and the operating system offering to repair it.
@physicalgoose:
Please try the following steps:
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Connect your OP-Z via USB, hold the screen button and turn it on.
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Open the OS X Disk Utility
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Select »[…] teenage OP-Z Disk Media« on the left side of the program
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On the right side, select the first tab named »First Aid«
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The program should now present you two buttons »Verify Disk« and »Repair Disk«. Use »Very Disk« to let Disk Utility check the OP-Z partition for errors and »Repair Disk« to solve them.
Let me know if it worked for you.