BR-8 only records two at a time as well. And I would NOT consider a ZIP disk format compared to a solid state SD card. Zip's are notoriously unreliable, especially after they have been used a while.
Is the BR-8 a new unit? Or has it been used..? Zip stores 100MB of sound data. SD card go as high as 12MB in the BR532...
Personally, if you want to start multitracking, I'd get an interface and start using your computer. Far easier, more flexible, and WAY more than eight tracks...
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An arranger is just a tool. What matters is what you build with it..!