Peter, it's hard to say anything about the real lifespan of CDRs and CDRWs right now - you hear claims between 1-2 years and a 100 years for CDRs, and most often something around 30 years. Nevertheless, I would prefer CDRs for backup, and CDRWs only for temporary "scratch backups", like raw scans, audio dumps, etc. etc. The density of the "writing" is much weaker on CDRW (that's why old CR players can't even read them), so I would expect a much shorter lifespan for them. But (and this is a big "but"), it is a known fact that the writing precision gets worse on the outside of discs, meaning that the chance to lose data gets bigger the more you write on them (all CDs are written inside->outside). Every test (I've ever seen) strongly recommended not to fill CDs up to the limit if it is supposed to be a precious backup, they typically recommended to use about 500 MB maximum, and to avoid the 700MB CDRs completely (for backups). On Mac you will probably use Toast, make sure to turn on one of the "optimise" options - this way the whole volume shrinks to its minimum size on CD. Without it, the blocks will be left where they are, and the CD will be filled "to the fringe" even if contains much less real data.