First error reports from my disk tests (see posts in the past few days) by checking :1) power supplies: two of my external disks (LaCie) were showing time-outs when I tried copying the data. I tried tons of things on them. Basically, the power supplies of both them were kaput (these were my older 2 external disks). I changed them (at a HEFTY price), and I got back my data with no problem whatsoever. So always check the power supply of your disks and try that as a cure first. There's no way to know this unless you have a spare GOOD one around (from another disk. Keep in mind I had 2 that didn't work). And, after contacting LaCie support, they sent me a spare one... but I had already bought 2 new ones. That might help (contacting support). Now, one of the disks is showing difficulties again, but I lost no data; I guess the lifetime of that disk is gone.
2) S.M.A.R.T statuses: I didn't manage to recuperate data on one of my other disks, although I tried everything (new power supply, Data Rescue). It seems to me that the disk is physically damaged. There is a standard, called S.M.A.R.T. that can alert you to this. The problem is that you can't read the status when you're attaching them with an external fireware cable (400 or 800), USB cable or eSATA cable. I "hacked" my MacPro by using 2 unused internal SATA ports, to an external port (in the USA get it form MacSales, in France from Macway). And by booting up the disk first (always with this case), then the computer, you get to read the status as if the disk were an internal disk. And guess what? S.M.A.R.T. status = FAILED !!! There was indeed a problem. A total surface scan also shows this. LaCie online support are willing to repair / exchange the disk, as long as I pay for one way shipment. Probably a good choice, as it will cost me only 10% of the price of a full new disk. Still, it sucks, as the disk is only a few months old.
3) bad blocks: Most disks have a few bad blocks. The only way to deal with them is to zero-out totally the disk with a full erase (Disk Utility), so that whenever the OS can't write to a block, it will map it out in the file system table. It's a must-do step for every new disk.
4) drivers updates: finally, I've been trying to use my LaCie eSATA cards to pilot my external disks. I've been having regular kernel panics; I contacted support for an updated driver. There is one (2009), that is not indicated in the list of drivers for the PCIe card (2008 only). The bad news is however that it is still unstable, and will have my MacPro crash whenever I do something fancy such as a RAID strip with external disks. I've nailed it down to the card and its driver, since connecting the same disks with only Firewire cables, or with my internal SATA extender (see 2)), doesn't produce this problem. It seems the only way out here is to use a great eSata card from someone else : Firmtek or Sonnet, hopefully one that doesn't need any driver (plug & play).
Bottom line, test extensively your disks when you FIRST get them (zero them out with Disk Utility, then surface scan them with Drive Genius to check everything is OK) to prevent any data disaster in the future. And set up a slave backup disk (also tested) for your master disk. TimeMachine is good enough for this.