Data Backups?

I assume Codger is talking about a drive enclosure that does RAID1 - mirroring two drives, the enclosure keeps the data the same on both of them. This is one cheap one: 3.5in Black USB 2.0 to Dual SATA Hard Drive Enclosure with RAID | StarTech.com Canada

RAID5 that Wickie mentioned is generally NOT used on servers any more. The basic idea is great - put 4-6 identical drives in a box, and the RAID system spreads the data out over all the drives so that if one fails, you can replace it and keep going. But with modern large hard drives, if one drive fails, it can take a looooong time to rebuild the data from the remaining drives. If, during that rebuild, there's another error, you can lose everything.

RAID6 is pretty much the same as RAID5, but with the data spread over an additional disk, so you can survive 2 failed drives.

Hard drives do fail over time, turning them off and on (spinning them up and down) is generally considered to be worse than leaving them running all the time. One common problem is when you take a drive that's always worked fine, and put it away as an archive. It sits in an acceptable environment for a couple years, then you plug it in to pull some data off and find it doesn't work.

My first thought for why you can't access data on an external drive on one PC would be permissions and/or admin access on one of the machines.

Other than expensive enterprise-level backup software for servers, I don't trust most tools to deal with incremental or differential backups. I use SecondCopy (other tools can do the same) to sync files between drives. Whatever drive I am using as the master, I replicate all the files to the other drives. I don't archive old files onto any other drive, or DVD or anything. All my files are on my master drive and I have a bunch of duplicates of that drive. It's the simplest thing I could come up with.

I believe that fairly lightly used thumb drives are safe for a long time. If you are writing and updating a lot of data on them, there's possibly an issue with them "wearing out." The memory cells can wear out, but it takes a lot of repeated write/erase cycles to do it. On SSDs, the firmware moves data around to avoid it, but as far as I know thumb drives don't do a lot of this "write leveling."

CD-Rs and DVDs are the most problematic, in my opinion. First of all, you have to get high-quality discs if you want to have any hope of them lasting. Then you have to hope that the new optical drive you get in 5 years will still be able to read the discs. I have some old CD-Rs that were very high quality disks, but written way back on a 2x CD-R drive... By the time drive speeds were up to 32x, the old disks were almost unreadable.

The iomega was connected to power but not to the computer and the power was off, not in standby. Suddenly I could hear it start and stop a couple of times and when I tried to get it working it was dead!

I searched on the net and found it has happened to others as well, probably some bad components. It may be possible to take the two disks out and fit in another drive or a computer to restore the data, but I have not done that yet.

I have a "Time Machine" now that is doing a backup mirror every hour and in between I am backing up files between laptop and desktop computers, but I am thinking of getting an SSD device that I can store elsewhere.

For 3.5" drives (normal desktop size), most manufacturers have an "enterprise" class that's rated as more reliable (and they cost more) than "consumer" level. If you buy an enclosure with a drive in it, you get whatever drive they put in there, often a consumer drive, but not always. It's possible to buy an empty enclosure and put your own drive in it, I've done that in a couple cases and bought Western Digital's more reliable RE drives instead of Caviars.

I don't have experience a lot of 2.5" drives, but I know the one I use as my master drive is almost 4 years old and has survived being carried around almost every day. (but in an anti-static envelope inside a padded case). Many (I would guess most, but I'm only really familiar with WD) 2.5" drives are designed to cope with the rougher use they'll get in a laptop.

Absolutely. The read/write heads are almost touching the drive surface when they're in use, and even though they're very very light, a bump can cause a head crash and loss of data. Lots of people (including me) have killed drives by knocking them off a desk when they're being written to.