IMO
• Multiple Libraries are self-defeating to the concept of an images database because they prevent global searches like keyword searches. Only for very specific and unusual reasons (like specific-client security of some kind) should there be more than one Library. Creating multiple Libraries for organizational or drive space purposes is misguided thinking.
• Large Managed Libraries are a bad idea for the reasons you are experiencing. They can be done well, but only by true mass storage experts with sophsticated large-drives setups.
• The Library on an internal drive with Masters/originals referenced on external drives is preferable for all but a small Library.
From a previous post of mine:
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The Library with its Previews lives on the internal drive and is always accessible. Masters live on external drives. The Library is backed up via Vaults and originals are backed up to redundant locations using the Finder before import into Aperture.
Personally I have images managed on the internal SSD until editing is complete then convert to Referenced-Masters.
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Database Physics
Aperture is designed to bite into small chunks at a time, so TBs of data do not bother the app itself. However, handling super-large batches of data like 1.5 TB on consumer hardware tends to be problematic.
Slower speeds seem to exacerbate handling large data chunks.
IMO referenced Masters make far more sense than building huge managed-Masters Libraries. With referenced Masters one has no need to copy a 1.5 TB sized file. A I find that even (2011 MBP) copying 5-15 GB-sized batches of RAW/JPEG files copying fails with some frequency, enough so that I always verify the copy. Failures copying 1500 GB to a drive as a single file should be expected based on my experience.
• Hard disk speed. Drives slow as they fill so making a drive more full (which managed Masters always does) will slow down drive operation.
• Database size. Larger databases are by definition more prone to "issues" than smaller databases are.
• Vaults. Larger Library means larger Vaults, and Vaults are an incremental repetitive backup process, so again larger Vaults are by definition more prone to "issues" than smaller Vaults are. One-time backup of Referenced Masters (each file small, unlike a huge managed-Masters DB) is neither incremental nor ongoing; which is by definition a more stable process.
Managed-Masters Libraries can work, but they cannot avoid the basic database physics.
Note that whether managed or referenced, original images should be separately backed up prior to import into Aperture or any other images management application. IMO after backing up each batch of original images importing that batch into Aperture as a new Project by reference makes by far the most sense. Building a huge managed Library or splitting into multiple smaller Libraries is less logical.
HTH
-Allen